[deccan]

I think this is really quite a cool idea. But I agree with the others that it seems like mostly a defensive measure.

Some points:

1) Vampires reincarnate in the capital, which can be a long way from the front in a big map.
2) When vampires route, you need to gather them up from neighbouring provinces again which wastes you time.
3) In theory, it is true that if you continue to whittle down an enemy army while your own vampires reincarnate again and again, even if you lose an indefinite number of battles, you will eventually win. However, in reality, there is no indefinite number of battles. Your enemy only has to win a limited number of battles in order to take your capital, and then your vampires can no longer reincarnate.
4) IIRC, having a vampire lord summon a vampire takes up his action for that turn. Having a devil generated from soul contract is automatic. So you could have the guys will soul contracts travel with the army and it would grow over time.

However, I still like the idea of a vampire army a lot. I'd personally like to see a White Wolf type vampire nation mod.