Are you asking about Skyrim orcs generally or specific to Dawnguard? In Skyrim Orcs like Tolkein's ones are a corruption of Elves in some sort of genetic imbalance or breeding plan. However there the similarity ends. They are insular and hard to kill, have starting bonuses in heavy armour smithing and 2h weapon. I don't really see the advantage of using them in Dawnguard unless you have an already developed orc character with stealth and similar stats etc, as warrior type skills skills don't look to be particularly useful to a vampire. Not so of course for maybe a Vampire Hunter? paladin type character? In either case you are I guess going to need some fire resistance so maybe the obvious choice is a dark elf
Yeah, I would agree with Tony about them being more probable to be vampire hunters, with their skills and bonusses, but if they have to be a vampire or werewolve, I would say they would be werewolves, as they love a good wild fight and wath would be better than a big supernatural wolf with that can kill an average person in one blow and kill them in a very bloody way.
I think that the personality of most Orcs would be more fitting with a werewolf. They Love a good fight and most magics and stealth combat kinda turns them off. They believe in the honorable way of fighting with their hands and determination. Vampires are more of the sneak up and stab you in the back kind of monster. not very fitting for an orc.