Okay, more fleshed out ideas on how the vampires will work.
The idea is to show a segregated society, with vampires ruling masses of thrulls.
The first melee unit is the bloodpet. Warrior replacement, starts with the thrull promotion.
Thrull promotion: Can be eaten to restore ability to attack and one movement point. Cannot become a vampire.
These upgrade to moroi. Axeman replacement, starts with thrull, can cast burning blood itself (preferably with only 25% chance of dying).
Now the line splits between the thrull units and the vampiric units.
Moroi can upgrade to Guard Thrull, a pikeman style thrull which start's with the "blood guard" promotion.
Bloodguard: unit gets +25% strength when defending a stack with a vampire in it. Automatically defends before any vampires.
Moroi at level 4 can upgrade to Brujah, a maceman replacement. However, this unit loses the thrull promotion (and doesn't start with it), so it can become a vampire but doesn't give extra eating bonuses.
Moroi at level 8 (quite high) can upgrade directly to vampires, at which point it loses the thrull promotion (duh).
Each of these only has one possible upgrade.
Guardthrulls upgrade to Calabim Sheildwall. It is a bloodguard thrull, but also starts with defender (just making it obvious). However, if upgraded from a moroi, then it would be able to burningblood itself. Burning blood temporarily deactivates defender.
Brujah's at level 8 upgrade to Aeron's Chosen, a berserker replacement. Not a thrull, and has the "Vampiric chosen" promotion. (do this without a promotion if possible).
Vampiric Chosen: When this unit dies it has a 4x% chance to be reborn as a vampire (keeps promotions and level, is born at half HP in capital). x is the unit's level.
The vampire and vampire lord (limit of 1 per governor's mansion perhaps?) are included as is.