Their rivalry would get in the way, as well as their pursuit for power. Goku would probably use Light and Vegeta would use Darkness. With his new powers, Vegeta might try taking down Goku to show he's the greater warrior. As seen in Revival, Goku lets his guard down too often while Vegeta's arrogance is his downfall. If the could overcome those faults and work together, they could take on anything. But alas, their characters are not of that nature. They would lose by the very characteristics that make them great.