is it possible for 1 of the 7 dwarves u start with to be a vampire?
No. And if anyone accuses one of the original 7 of being a vampire on the justice screen the accuser is usually a vampire trying to shift the blaim according to a posts by ThreeToe on the main page.
I've got dwarves in dwarf therapist that don't appear in the fortress too. Don't think they are vampires though. They are both listed as master spear dwarfs (with no spears skill) and their labours are unmodifiable (indicating they are nobels?). I have no military dwarfs in squads at the moment.
So far I've had 2 vampires out of 150 dwarfs. I don't think that's an unreasonable number, but their frequency is clearly tied to world gen and how many vampires are available as historical figures to migrate to your fortress. If you reduce the number of years you let the history run you can be sure that will reduce the vampire frequency.