It is but Dhampyrs can't become Vampires, so they could only ever pass on the trait, not use it themselves to unlock the Occult. I'll add a safeguard so that the trait isn't wasted if a player tries to unlock it anyway, though, since you did just make me realize trying wasn't blocked for them, oops.
Dhampyrs can still become Occults from other non-Vampire Dormant traits, though.