Clearly, Alucard has immigrated to your fortress and is on watch for other, lesser vampires. Weaponize him.
The "has the appearance of someone x years old" is normal and means he was alive at the start of worldgen and was created by Armok already grown up, to avoid a planet of children in year 5. All adult dwarves will have that in their description on short-history worlds. Vampires in my (old) world impersonate dwarves who were born after worldgen.
How exciting! I'd never seen that message and did not know how to interpret it, thanks for explaining this.
Trying to give him a nickname is the only reliable way to test if a dwarf is normal or not. The inventory screen lies on vampires and shows just regular clothing with no jewelry. The "is a former member of" list is NOT a lie on vampires and will often be extremely long because they've spent centuries going from one civilization to another to feed.
I disagree about that being the "only reliable way". I noticed a slew of other unique characteristics of this guy, some of which are highly obvious, others are more subtle...
He goes on break only to feed. Interrupt that feeding and he goes back to work immediately - let him finish the feeding and he'll continue with a more normal break.
He does not claim a bedroom. I don't assign everyone bedrooms, I make bedrooms and let dwarves autopick as they get sleepy - that's about a couple of monthsish after getting to the fortress. If you attempt to go to the screen where you can assign bedrooms and look for who hasn't got one - there's your children and vamps. (wouldn't work to find a child vamp of course).
His thoughts are far, far more limited than other dwarves. Granted, my guys are getting next to no idle time - but all the thoughts related to the normal processes of life, which fill all my other dwarves heads within a couple months of immigration, are absent.
In Therapist, his name is absent, but there is a different name with skills and personality traits that match his. Additionally, both of my vampires claim to ardently worship the god that cursed them - and the name on therapist matches the name they gave (and their fake names are nowhere on the list of that god's followers).
I wouldn't say that their membership list is excessive, but they do have one eerie membership that no one else does - both are "former sacred mists" of a civilization branch (surely a cult of some kind).
My vampire that claims to be married with kids immigrated without his wife or children - the only dwarf in my fortress to date to have arrived 'alone' after appearing to have a married family (that seems to be why we are awash in kids - every underage family member seems to migrate with the parents, who seem to migrate together)
I've had immigrants get retrieved before, but I'm unsure of why. They could have spawned on a ledge or something or, depending how in-depth worldgen goes, could have been wounded at the time your fortress starts. Show up, it's been a little while, game says that it's been long enough that they heal but hasn't clicked off the wounded tag yet.
All immigrants seem to spawn on the same tile and walk inwards... there's no ledge I can see that could injure any of them. However, I bet it did arrive wounded and heal - except that its body shows no scars, and as far as I've seen (though this is all from DF2010 so far) if a dwarf gets hurt and doesnt get medical aid, they always scar - then again, vamps may be very different.
For anyone wanting to see, here's the relavant screens for anyone wondering about these dwarves and their thoughts/memberships/relationships and so on.
vamp 1, the 'normal one':
vamp 2, the 'carry me' one:
I'll be REALLY interested if I get any other vamps, because it appears on surveying my fortress that only one of my fortress's gods curses, and he only made 2 vamps.