Best combo!

I need combo ideas what could beat a vampire in one turn with one up to 17 level player?

At level 17, dealing 144 damage to a single creature who's resistant to nonmagical and necrotic damage seems not too crazily difficult...

I've never played to such a high level, though my party is approaching it steadily. I suspect you'll be looking at a Sorcerer. At 17th level ,you'll have a 9th level spell, which includes Power Word Kill, meaning with Quickened spell you need only deal at least 45 damage and then cast Power Word Kill to wipe it out.

I'd suggest going for:

1: Power Word Stun, to stop the Vampire
2: Delayed blast fireball (quickened)
3: Wait for them to break free of the Stun, and at that moment, end the fireball spell.
4: Power Word Kill to finish them off.

Or, for true one turn,

1: Disintegrate
2: Quicken spell, Power Word Kill.