Namora is very skilled under the water. Saiyans can’t breathe when submerged so she could maybe win if she keeps Vegeta down there long enough, but it’s not the most practical plan as he could just break the Earth underneath it to drain the water. Vegeta is effectively invincible so defeating him is simply out of the question for most fighters. I’m afraid that Namora would not be able to do all that much here. Vegeta wins.