I'd probably have them come back as a Vampire.
Their hp maximum is 0 so they can't be alive, so if they come back it'll be as undead. I guess the kindest would be to let them return as a Revenant.
Why would they come back as undead? If they were undead, raise dead won't work. The vampire power is pretty specific: "A humanoid slain in this way and then buried in the ground rises...". So yes, they died, but unless they're buried they don't come back as a vampire.
EDIT: In addition, gentle repose specifically states that while the spell is still in effect the corpse cannot become undead.
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