Being a foreign word ending in a non-Greek ending, there is no plural. In Modern Greek, if a noun ends in something other than a vowel or sigma, it can’t be declined. (Nu is archaic; rho xi psi even more so.) So το βαμπίρ, τα βαμπίρ.
I see that at least one person online has named themselves Vampirissa “vampiress”, forming a feminine of the form. But that would be considered informal usage: formal usage would just put a feminine article in front of βαμπίρ.
There is a reason unassimilated loanwords suck.