Most astute readers will quickly notice that this build is doing (at least) TWO things that Shadow Priest builds rarely tend to do (in the modern era).
- Threat Reduction talents (plural!)
- Improved Vampiric Embrace
Another thing that this build does which is almost never seen on other Shadow Priest builds (that I’m aware of) is …
- Skip Darkness entirely
… although an explanation for why that is so will have to come a little bit later, in due course. 
The typical reason for not investing into Improved Vampiric Embrace is simply a matter of Threat management (and competition for scarce talent points) on builds that commonly eschew ANY effort at Threat reduction. The easiest way to explain this point is to do a simplified Analysis of Alternatives and let the Threat generation fall where it may. 
For simplicity of math (and analysis) … let’s say that we’re dealing with Shadow spells that deal 100 (base) Shadow damage on hit. 
- The rate of Threat generation from damage is 1 Threat per 1 damage.
If you’ve got (base) Vampiric Embrace on the target, you’ll be getting 20% of the damage dealt back as healing. Each talent point spent into Improved Vampiric Embrace increases the amount of healing return from Shadow damage by +5% (so +10% @ 2/2).
- The rate of Threat generation from healing is 1 Threat per 2 healed (overhealing does not count for this!).
What you wind up with is a relatively straightforward computation as a soloist (where Threat doesn’t matter, YET…).
- 20% Vampiric = 100 + (20/2 * 1) = 110 Threat for 100 Shadow damage
- 25% Vampiric = 100 + (25/2 * 1) = 112.5 Threat for 100 Shadow damage
- 30% Vampiric = 100 + (30/2 * 1) = 115 Threat for 100 Shadow damage
The problem occurs when you start getting into dungeon groups or other 5-man party contexts.
- 20% Vampiric = 100 + (20/2 * 5) = (up to) 150 Threat for 100 Shadow damage
- 25% Vampiric = 100 + (25/2 * 5) = (up to) 162.5 Threat for 100 Shadow damage
- 30% Vampiric = 100 + (30/2 * 5) = (up to) 175 Threat for 100 Shadow damage
Needless to say, at these rates of Threat generation, you’re going to be “competing” with your main tank for aggro … which in a 5-man context isn’t necessarily the best thing you could be doing (see: Cloth Tanking). Point being that Vampiric Embrace is quite the double edged sword when it comes to Threat generation and the management of it.
I get the sense that the Blizzard Developers expected Players to do what I’m doing in this build … reach for the Threat reduction talents to mitigate the “magnifier” effect of Vampiric Embrace … and then watched with bemused disappointment when those same Shadow Priest Players laid their aggro management woes at the feet of the “inadequate” tanks, rather than on their own choices. Why would that happen?
Well … 
Remember kids … Personal Responsibility™ is only for OTHER PEOPLE … not YOU! 
Vampiric Embrace is a pre-requisite requirement to get Shadowform … but the mitigation of the Threat that Vampiric Embrace can generate IS OPTIONAL … and since Threat Reduction generates ZERO DPS … 
Besides, Threat has zero meaning in PvP … and Shadowform Priests MELTING FACES is about PvP … so, bye bye Threat Reduction in Shadow Priest talent builds! 
Therefore, Threat Reduction talents are only “relevant” to PvE builds. 
So what happens when you stack Threat Reduction talents together? 
You wind up multiplying their reductions, rather than straight adding them.
20% reduction = (1-0.2) = 0.8x
25% reduction = (1-0.25) = 0.75x
20% + 25% reductions combined = (1-0.2) * (1-0.25) = 0.6x
So using that understanding, let’s circle back to our original Threat generation values for using Vampiric Embrace (assuming no overhealing, to keep things simpler).
Solo: -20% Threat Reduction (5/5 Silent Resolve)
- 20% Vampiric = 100 + (20/2 * 1) = (110*0.8) = 88 Threat for 100 Shadow damage
- 25% Vampiric = 100 + (25/2 * 1) = (112.5*0.8) = 90 Threat for 100 Shadow damage
- 30% Vampiric = 100 + (30/2 * 1) = (115*0.8) = 92 Threat for 100 Shadow damage
Solo: -25% Threat Reduction (3/3 Shadow Affinity)
- 20% Vampiric = 100 + (20/2 * 1) = (110*0.75) = 82.5 Threat for 100 Shadow damage
- 25% Vampiric = 100 + (25/2 * 1) = (112.5*0.75) = 84.375 Threat for 100 Shadow damage
- 30% Vampiric = 100 + (30/2 * 1) = (115*0.75) = 86.25 Threat for 100 Shadow damage
Solo: -20% Threat Reduction (5/5 Silent Resolve) AND -25% Threat Reduction (3/3 Shadow Affinity)
- 20% Vampiric = 100 + (20/2 * 1) = (110*0.6) = 66 Threat for 100 Shadow damage
- 25% Vampiric = 100 + (25/2 * 1) = (112.5*0.6) = 67.5 Threat for 100 Shadow damage
- 30% Vampiric = 100 + (30/2 * 1) = (115*0.6) = 69 Threat for 100 Shadow damage
Where this gets interesting is when you move from the Soloist context (where Threat doesn’t matter) into the 5-man group context (again, assume no overhealing for a “worst case scenario” dataset).
5-man: -20% Threat Reduction (5/5 Silent Resolve)
- 20% Vampiric = 100 + (20/2 * 5) = (150*0.8) = 120 Threat for 100 Shadow damage
- 25% Vampiric = 100 + (25/2 * 5) = (162.5*0.8) = 130 Threat for 100 Shadow damage
- 30% Vampiric = 100 + (30/2 * 5) = (175*0.8) = 140 Threat for 100 Shadow damage
5-man: -25% Threat Reduction (3/3 Shadow Affinity)
- 20% Vampiric = 100 + (20/2 * 5) = (150*0.75) = 112.5 Threat for 100 Shadow damage
- 25% Vampiric = 100 + (25/2 * 5) = (162.5*0.75) = 121.875 Threat for 100 Shadow damage
- 30% Vampiric = 100 + (30/2 * 5) = (175*0.75) = 131.25 Threat for 100 Shadow damage
5-man: -20% Threat Reduction (5/5 Silent Resolve) AND -25% Threat Reduction (3/3 Shadow Affinity)
- 20% Vampiric = 100 + (20/2 * 5) = (150*0.6) = 90 Threat for 100 Shadow damage
- 25% Vampiric = 100 + (25/2 * 5) = (162.5*0.6) = 97.5 Threat for 100 Shadow damage
- 30% Vampiric = 100 + (30/2 * 5) = (175*0.6) = 105 Threat for 100 Shadow damage
And just for reference, so readers don’t need to scroll the page …
5-man: zero Threat Reduction
- 20% Vampiric = 100 + (20/2 * 5) = 150 Threat for 100 Shadow damage
- 25% Vampiric = 100 + (25/2 * 5) = 162.5 Threat for 100 Shadow damage
- 30% Vampiric = 100 + (30/2 * 5) = 175 Threat for 100 Shadow damage
The differential in Threat generation between minimal and maximal cases in a 5-man context then becomes:
- 20% Vampiric = 150 - 90 = -60 Threat per 100 Shadow damage
- 25% Vampiric = 162.5 - 97.5 = -65 Threat per 100 Shadow damage
- 30% Vampiric = 175 - 105 = -70 Threat per 100 Shadow damage
It also means that you can build for two (comparable) variations on the Shadowform theme when concerned with Threat generation:
- NO Vampiric Embrace, NO Threat Reduction = 100 Threat per 100 Shadow damage (5-man)
- 30% Vampiric Embrace, 60% Threat Reduction = 105 Threat per 100 Shadow damage (overhealing will reduce the Threat generation below this amount)
So a “silent vampire” generates functionally equivalent Threat levels to those that a “loud non-vampire” Shadow Priest build will (the difference is in “rounding error” territory).
Then why would any Shadow Priests CHOOSE to be “loud” rather than “quiet” on their Threat generation? 
Because choosing the “silent” option IS NOT FREE. 
It actually requires allocating (up to) 10 talent points in places that you would rather NOT spend them into (if all you care about is “MAX DPS NAO!”).
- Silent Resolve - Rank 5/5
Reduces the threat generated by your spells by 20%. - Shadow Affinity - Rank 3/3
Reduces the threat generated by your Shadow spells by 25%. - Improved Vampiric Embrace - Rank 2/2
Increases the percentage healed by Vampiric Embrace by an additional 10%.
Talent points “spent” there cannot be invested into Improved Mind Blast, Shadow Reach and/or … Darkness … all of which are “more obvious” choices for creating MAX DPS opportunities, so why self-gimp your way out of BIG NUMBERS? You became a Shadow Priest to MELT FACES in PvP, didn’t you? 
Well … some people do … but not everyone … 
There’s also a curious overlap going on here with the Drain Tank Warlock playstyle which opens up when reaching for the “silent vampire” option as a Shadowform Priest.
The idea with a Drain Tank Warlock build is a combination of “What’s mine is MINE, and what’s yours is MINE TOO!” thanks to (Shadow damage) DoTs and Drain/Siphon Life (with Dark Pact being optional) combined with Fear. You basically “juggle” mobs and use multi-DoTs to keep yourself flush with green+blue bar resources.
The thing is, while that strategy is quite viable … it’s also inherently selfish.
A Warlock can’t “share” the yield of their Drain/Siphon Life spells with others. Only the Warlock benefits (personally).
A “silent vampire” Shadowform Priest can achieve similar results to the Drain Tank Warlock … but do so in a more “group friendly” fashion against dungeon trash. 
- YOU get (Improved) Vampiric Embrace … and (Improved) Shadow Word: Pain!
- YOU get (Improved) Vampiric Embrace … and (Improved) Shadow Word: Pain!
- YOU get (Improved) Vampiric Embrace … and (Improved) Shadow Word: Pain!
- YOU get (Improved) Vampiric Embrace … and (Improved) Shadow Word: Pain!
- YOU get (Improved) Vampiric Embrace … and (Improved) Shadow Word: Pain!
EVERY MOB in the spawn group gets DAMAGE that HEALS the 5-man Party almost continuously! 
It’s like you’re a Drain DPS Warlock Priest who “heals the whole party as a side effect of what you’re doing” … while the main tank has less to worry about and any Warlocks nearby will be Lifetapping hard enough to want your babies. 
And if you’re a Troll Priest, you also get Hex of Weakness (ranks 1-6), a highly underrated debuff which can make mobs deal less damage AND reduce the effectiveness of any Healing they might receive. You’ll also get Shadowguard … which does Shadow damage any time a spell, melee or ranged attack hits you (which includes things like Detect Magic! 
) and is among the most efficient ways for you to convert mana into Shadow damage.
Stack Intellect+Spirit gear on a Troll Priest and watch your Regeneration blend into the mix as additional mitigation for damage taken, reducing the need to drop Shadowform in order to heal yourself. 
Need more mana? Get the killing blow on something (worth XP) for a Spirit Tap proc. 
“Fear me. I’ve killed hundreds of champions!”
“Fear me. I’ve killed ALL OF THEM …”
