I would think it would be one on one... otherwise, Xemnas has the Org, that Dragon thing, and an endless supply of Nobodies. Not to mention two of himself. lmao.
Psh. Think about that. Vader is just good with swordsmanship... and he can move/shock/choke things with the dark force. But come on. Xemnas is a million times faster, and can do the same things... but far greater. Vader has one light saber. If that's gone, he's done... Xemnas can use two at a time and summon them as arrows and everything. Then there's cloning and his god-speed...