If Vegetto goes serious from the start, he'd win with a large enough attack to completely destroy Boo, otherwise Hit's improvement ability + time leap + vital point strikes causes Boo to win within a few minutes. Even in the case of the former scenario, it's also possible for Boo to negate Vegetto's attack with Botamo's ability, though I imagine Vegetto's Beam Sword Slash may be able to do something due to being a technique specialised in piercing an opponent.