I'll be damned, I started a DK group yesterday to test out the auto interact, which is amazing.

I havent really tried anything myself, but I think your Theory Crafting is right. Plus, add in the healing of blood aura in general, which is 4% of your damage, plus the 6% of hp healed after every kill with vendetta and you do have a very strong, helaing based build.

Even if you don;t activate, vampiric Blood you still recieve 11%, from rune tap, 11% from imp rune tap 10% from rune tap AoE, time four, plus the 10%heal increase of imp blood aura, so it's 68.2 % without vampiric blood.

With vampiric blood, and increase of 35%, you get a whopping 92.07% of your hp recieved back. Really, if you activate this you team is gonna be at full health, anything lower than 10% this isnt true for but you will probably use this before that, also you can indeed macro it because Vampiric blood is NOT affected by the GDC as far as I know. Generally It could be awesome for pvp. Combined with Mark of blood on an enemy you could probably use this and that to stay at full health for a good 10 seconds while you beat your opponent down. Or chain it if your going up against a single, slow hitting, high hp enemy like a tank.