I was thinking of turning my Breton into a Vamp Spellsword, but only after killing of the Volkihar so Serana an him can be the last pair in Skyrim.

What do you guys think? any ideas about how I should go about it?

I was thinking of something like he kills the Volkihar and realizes that he can help the world better as a vampire, as he can live longer and appear as the world needs him, or something like that.