I'd suggest that it might be Awakened, but I doubt it... Belkar's wild empathy wouldn't work on it, and it would have been able to talk when it was a dinosaur.
It might still have its original HP and HD if it passed the will save (unlikely) but that wouldn't boost its attack... Yeah that's super weird actually, how the hell did a lizard overcome a vampire's damage resistance? They do 1d4-4! (So, 1)
...Basic T-Rex (which I assume this is based on, even though the wiki insists it's an allosaurus) has one level of Improved Natural Attack (Bite). If it passed the will save it still has that, bringing its bite up to... 1d6-4. Vampire spawn have DR 5/silver (full vampires have 10, but maybe this priest's just a spawn). Bloodfeast could overcome the DR with two more levels of Improved Natural Attack, which is actually possible without class levels AFAIK. I think animals still gain feats every three levels, they're just limited in what they can choose. So instead of being an 18HD T-Rex, it'd be 24HD or thereabouts.
If it reached 37HD it would become gargantuan! Wolves do this too... Normal 3HD ones are medium, but 4-6HD ones are large. Kinda surprising since humans are medium, and these aren't even dire wolves. Interestingly, dire wolves start at 7HD, kinda hinting that a 7HD wolf should become a dire wolf.