Hmmm...Interesting question...A Vampire is someone's worst day personified, but they're more then that...They're them if they never grew past their worst day. Essentially, A Vampire is "You, but Evil, and sans any character development." So, for instance, Vampire!Belkar would basically just be regular Belkar, but missing even the slight hints of good will towards others that he's recently started showing. Roy's Vampire would be, as OP stated, all of Roy's former frustration and anger towards his party, the kind that he normally uses sarcasm to cope with aed led him to do the most Evil thing he's ever done, namely, abandoning Elan to possible death. He'd...actually probably be a lot like Tarquin. A controlling jerk who punishes harshly or even kills anyone who refuses to fall in line. Kinda like Roy in this strip, but to a far greater extent.