So what..maybe he inherited some of the Vampire's increased strength and speed but maybe not all of it? Enough to put him at an advantage over most mortal foes? I can kinda buy that. Combined the the natural racial bonuses of the Orc, and their Berserker rage, I could see how a half vampire could tear through most opponents.
That's what I always assumed.
I actually wonder about species, temporarily setting aside the vampire debate. Mostly because of Bretons, who are basically multi-generational half-elves. In the real world, the certain way to determine if two things are the same species is if they can produce fertile offspring. Elves and humans clearly can, but do the same rules apply? This is a created fantasy world, after all, so there's no guarantees whether our science is their science. Now, back to vampires, we'd need to have Agronak get laid to tell for sure, then.