You'll need different gear setups, CP distribution, and likely different weapon types. If you're soloing a Magplar in PvP you'll likely want to be a vamp. You can do this all on the same character, CP respec is only 3k gold and you can feed if you decide to go vamp, but I guess it would depend how often you'd go back and forth between the two. If it's almost daily then you might want to roll a new character.