Dear Ms. Slayer,
Is it possible for a vampire to not know what it is?
Not only is it possible, it's practically guaranteed. A vampire cannot see its own reflection in the mirror, remember? You probably know this if you've ever tried to get your vampire to look at itself -- remember when your vampire was listing all your faults, shortcomings, weaknesses, etc., and you finally piped up and said, "Hey, what about YOU? What about your faults, shortcomings, weaknesses, etc.?"
Did you have any luck with that?
I'll answer for you: No.
Now you answer: Because a vampire cannot see its own reflection, that's why.
Those are the rules.