Vegeta. It's not even a contest. Let me put this in perspective for you. Goku could see and dodge individual machine gun rounds and shatter boulders with his fists when he was a kid. He only got stronger as the series progressed and at the start of the Saiyan arc was still considered laughably weak compared to Vegeta and Nappa. At that point Vegeta against Harry would be a mismatch, and once again Vegeta and Goku only grew stronger, faster, more skilled and more powerful as the series progressed. To give you an idea of how laughable this got, the average human could be expected to have a "power level" of around 5 or so. Goku was introduced with a power level of 10 (see the aforementioned boulder crushing for perspective there) at the start of the Dragon Ball franchise, Vegeta was introduced with a power level of 18,000 and the capacity to destroy planets. One could very well argue that a Wizard is above the average of 5, but I don't think you can argue that they're 3600 times better than an average human.
Now, ignoring for a minute that speed alone puts the ball thoroughly in Vegeta's court and that the only way to bypass that is if Harry acts incredibly out of character, we also have Vegeta's enormously greater combat experience to consider, and the fact that nothing in Harry's arsenal should by rights be able to put Vegeta down. Harry does not use Avada Kadavera. He stopped using Secumsempra as soon as he realized what it did. Stupefy might work (though as a spell that can be tracked with the eye, it fails in practice due to the aforementioned speed issue), but the key word there is "might". For starters, various species in Harry Potter have shown a natural resistance to it which allows them to brush it off with hardly a moment's pause, including Hagrid the half-giant. Additionally, it's one of the spells that can be guarded against to negate its effects. And as a humanoid alien who routinely brushes off blows to the head that could flatten mountains and has an incredibly powerful 'ki' aura to boot...the idea that a spell designed to disable regular humans would affect him is questionable. Not impossible, but it cannot be assumed to be a given as it has never been attempted on anything remotely comparable and we know it lacks universal application.
To even give Harry a fighting chance we have to first assume that he's acting out of character, then assume that he has far greater magical knowledge than he was ever shown to possess, then ignore Vegeta's enormous physical advantage (particularly in the sheer speed department)...and it keeps on going. That's not a "who would win" question, that's a "how much can we stack the deck to make this a fight" question.
