Whoa, wait a minute. Couldn't you just put the syndrome into BLOOD_TEMPLATE in material_template_default.txt?
Dunno. Never thought of trying it. Looking at the creature raws it looks like the syndrome applies not to the blood template itself, but the pseudo-bodypart defined as blood and using the blood template.
I added the
[ENTERS_BLOOD]
[SYNDROME]
[SYN_NAME:has bitten a dwarf]
[SYN_AFFECTED_CLASS:GENERAL_POISON]
[SYN_INGESTED]
[CE_REMOVE_TAG:NO_DRINK:START:0]
stuff to the end of the blood template (the 'enters blood' line is unecessary, but I wanted an exact copy of the existing functioning syndrome). It didn't bring anything up in the errorlog, but feeding on a dog didn't give any syndromes beyond the existing unnamed vampire one. I tried this with the pre-existing dwarf blood syndrome and without it, and only in the former case did the vampire drinking her own blood transmit the 'no drink' syndrome.
On the plus side, since anything with blood uses the same line of text to define it, you could use grepWin or something to find the blood text and insert the syndrome text after it. Did this for a little chitin / scale mod.
Edit: Okay, something is really weird here. Putting that syndrome in the arena on vampires causes some odd behaviour. They'll get thirsty, which is fine. In that state, drinking from a dwarf will remove the thirst status from the adventurer-style interface, but not from the fortress-style interface. Drinking repeatedly from a dwarf doesn't seem to harm it, and drinking water eventally brings up the 'you are too full' message but doesn't remove thirst from the fortress-style interface.
The same sort of thing happens with a regular dwarf. The adventurer interface is reading hungry/thirsty/drowsy, and the fortress interface is reading starving/dehydrated/drowsy. There was a difference in that drinking water removed thirst from the adventurer interface, but all things considered I don't know how the arena performance translates into fortress mode. I've never gotten any vampires to test with, pre-mod or post-mod.
More editing!: I removed the 'enters blood' line for being mechanically unecessary and the syndrome name for being uneeded. I think this might have fixed things, or maybe they weren't broke in the first place. I let dwarves and vampires mingle for a while, then took control of one of the vampires. She was thirsty and had only the vampire syndrome, so no feeding had taken place. I fed on one of the dwarves and then ran around for a bit. The problem here is that there seems to be a mismatch between the two arena views, because if you wait for a thirst indicator when running around the vampire is going to die of dehydration as soon as you switch to the fortress view. So I flipped back and forth between the two modes until the vampire was thirsty in fortress view only, and then swam across the water and headed for the top of the map. Then I watched. She headed down to the dividing water, being thirsty the whole way, and after some time standing next to the water was no longer thirsty.
So, it seems to work? But now that hunger and thirst and drowsiness are back in adventurer mode, it does weird things if you take control of a creature in the arena long enough for it to be an issue (time runs differently in the two modes). Creatures you're not controlling will sleep or drink if they need to, but they don't seem to accumulate hunger or thirst when you're not controlling them.