Vayne is an absolute pain. I have yet to defeat him. It's impossible to beat him just by doing the main story, you have to level up by doing loads of side-quests or grinding. That would be okay if any of the side quests were worth doing. In my opinion, they're not. I gave the whole "hunting" thing a try and it was just too tedious. And as for grinding, the battle system was beginning to wear on me as I approached the end, so I just couldn't be bothered. I just watched a youtube video of him being defeated and the end cutscene instead.