If you like, I can rephrase that as "Any GM using the Bag of Rats rule". Obviously, you don't have to run the game by the rules, but that does break the warranty.
Well, that's pretty direct and a little inflammatory too, I think. But we will have to agree to disagree on this.
Because the whole idea of vampirism is that the Vampire can't create vitality from nothing; if she could, she wouldn't need the blood. Instead, the entire schtick is predicated on moving vitality from one place to another. That said, I'd forgotten that Blood Drain does do a decent chunk of damage to the target; as such it's only -really- broken if you capture a regenerator and carry them around.
To me I was seeing the prisoners and their blood as the "something" involved, and you are moving vitality from them to you. Granted, on a level of symbolism, beating them might wreck their vitality a bit. I see your point on this, and I think it just depends on where the line you want to draw in the narrative is.
I think BD specifies that it has to use a vampire at will as well, so it is damage on top of damage really. With that in mind, do you feel like the possible balance* problem could be resolved by requiring an out of tactical combat BD on a helpless foe to be a CDG that kills the target?
KInda. The thing is, even if it's not overpowered, it's worth noting BoR situations as they occur, as there are actually several reasons to honor the BoR rule:
1. It can create broken results.
2. It gives players a benefit for adding to the clutter of the game.
3. It lets players create repeatable powers out of nothing.
4. It uses powers in an unintended way.
What's a complicating factor is that in this case, the designer has said outright that he expected it to be used on helpless prisoners. But lots of powers produce unintended results if you let them be used against walls/rats/prisoners. So it's worth examining any tactic that starts out there very closely.
Right; if I thought that the combination was broken, I would have no problem houseruling it. I would agree with your guidelines; I just feel like the blood from the enemies knocked out constitutes something for number 3, and that the end effect isn't as breaking as it would need to be for number 1.
Number 2, though... The prisoner/blood drain think is pretty cluttery. There, I can agree. Maybe making some houserule that is essentially summarizing this mechanic.
"Lifeblood Scavenger - After a battle, while taking a short rest but before you regain your HPs and have your healing surges reset to normal, you can drink blood from the fallen foes and recover an additional healing surge."
That really seems more like a feat to describe the mechanic, which might also help with the feelings of balance, since it is requiring a cost from the player.
* Again, I am not sure there is a balance problem, but I think that developing the idea against others will make it run better. I believe it would work fine. However, I have believed in Santa Claus, the Easter Bunny, and that the Transformers home base was a big superchurch near where I grew up when I was 3 (it looked just like their base on Cybertron, I swear!). So I have been wrong in the past, I will be wrong in the future... and I might be wrong right now.