That's Goku's problem. Imagine if Vegeta was a legitimate enemy who tricked Goku to knock him out in the same way. Dude could have just blasted him into oblivion while he was unconscious. Luckily nice guy Vegeta let him live.
Secondly, holding back his full power isn't something Goku did particular to this fight. He does it all the time and he gets shit on from it from everyone. So while in isolation it might seem like he was doing it for Vegeta's sake, Galu has repeated that habit on multiple occasions. His friends warn him on the regular to get serious and go full power from the start and he never does. Here, he got TKO'd for it.
Thirdly, Final Explosion>>SSJ3 Galu, so the fight goes back to a draw in a worse case scenario for Vegeta
Yeah the thing is he wasn't. He wasn't a legitimate enemy.
He has literally been living with one of Goku's closest friends for 7 years, his kid is best friends with Goku's kid, I mean please let's stop with making this it out to be something it wasn't with the "all is fair in war" crap when you know it simply doesn't apply in this situation. "Lucky Vegeta nice guy let him live?" i mean you're just trolling there, right? In Goku's eyes, he is fighting his friend who just lost his mind, hence he trusts that considering the fight was formal, (Goku wasn't randomly attacked, Vegeta waited for him to accept), in the very same way, once they agreed to postpone it, it's expected to be respected and Goku didn't fail by "lowering his guard" , he did so in total acordance to what the situation was, which is: this was not an war against a villain, like the one in the Sayian saga, with no rules, other people joining in, etc. On the contrary, this was a fight where:
1st, Vegeta wanted Goku to be 100% concentrated in their fight, (was annoyed at Goku being distracted by the Buu situation), and 2nd, Vegeta wanted Goku to give his all (was offended when found out he hadn't)
These two points matter because they directly contradict the idea that Vegeta would care to defeat Goku when none of those 2 aspects are happening because Goku is NOT FIGHTING and NOT EVEN AWARE THEY ARE.
Which is why Vegeta's punch from behind wasn't a part of the fight, had nothing to do with defeating Goku or proving to be better. It was only to not have Goku interfiere as he wanted to take care of Buu alone and the next thing he already is asking Piccolo wether he'll see Goku in afterlife, meaning he's still hung up on beating Goku, why would he do this if he just accomplished the "win" he yearned for so long just minutes ago? Because he didn't win.
Whatever happened before or after the fight, has zero relevance to the fight. Again I repeat, what ever happened after the fight stopped by agreement, is irrelevant. You can dance around this all you want to make up for the fact Vegeta didn't win and yet he didn't win. To say he did is as absurd as lets say, if one day in the future Goku randomly knocks Vegeta and declares he won this same fight, only it was years after. Would you say that's a win for Goku? If the answer is no, what is the difference between Vegeta doing it moments after the fight had ended? Is it time? What would constitute a valid time frame within a vague enough concept of a fight in which "there are no rules" and "all is fair" ?
And just to consider the weirdly underrated point that Vegeta needed to be possesed by Babbidi just to be able to stand a chance, I mean he literally says this himself. That to me, disqualifies the fight as whole, Vegeta is cheating from the get go, he's not using his own power. He didn't achieve it naturally nor legitimately. I can only imagine if this was the other way around and Goku was the one who did this, we would not hear the end of it from Vegeta's fans, saying how Goku had to resort to tricks to compensate the fact that he was inferior in power, you'll be saying how it obviously doesn't count, you know you would! Please, it's a disservice to Vegeta to push the narrative that this was a valid fight for him that served to demonstrate he was better. It didn't. It demonstrated he is guiled and a fraud. Claiming he hates techniques and "magic tricks" and yet going as low as allowing himself to be possesed in order to gain what he said himself he lacked. If you find all of that to be "badass" and cool that's fine, but the fact remains Vegeta didn't win this fight and he knows it, which is actually honorable of him, and you should respect his opinion.