Although it may look it~ I am very un-bias, but I think Sebastian would take home the win =3 (Im pro Alucard and Sebastian...hell if anything they kids should team up 0-0)
But Sebastian has fought Death gods, angels, took a gun to the head, can mange to keep his emotions under control, and he didnt even need a firearm~ so I am thinking (no matter how bad @$$ the vampire is) vampires would be a breeze. I mean the guy lost an arm~ and didnt even bleed~ somethings holding the guy together. And the final reason being beacuse he is in a show pandering to fujoshi XD yea~ I went there (seriously jk =P)
...of course they both are immortal~ so maybe they both would win...or maybe neither would make it out alive.