Vegeta stated he'd be the one to win the tournament in the end... While he wasn't that confident, as he waited Goku to fight firstly and was willing to observe firstly, that likely came from him expecting Goku to have more power, as before Goku fought, he stated "show me what you can do". And he was also anxious for the battle, so much so he was the first one to arrive there, which Goku took that as a conclusion he got way stronger... As he'd have no reason to be this anxious if he knew he stood no chance, as we know how Vegeta is when he knows he's useless. So he was confident but still cautious, as he's dealing with a still unknown >>> 50% power from Goku that he expected and more than what Cell has shown up until there as well. But for part of his confidence to be validated, which he only lacked and admitted he couldn't surpass Goku and Cell when they showed their true power (well, still not Cell), ought to mean the least the knows from Goku, his 1/2 power, must have been surpassed. I think I generally have him at 66% of CGs Goku, and since Trunks was pretty much his equal, as Cell said both Vegeta and Trunks were finally fighting evenly with the Cell Juniors, then Trunks should win this. I'd say without many problems, though I'd also say they're closer to 50% Goku than to FP Goku. Like if 50% Goku were 30k Vegeta, [CGs] Trunks and Vegeta were Reacoom and 100% Goku were the ~60k Goku who schooled the Ginyus.