The vex don't have morals they are machines, there's nothing skewered about it. With everything we know based on playing destiny and grimoire and assuming you've seen the anime, watched the movies and read the manga then we also know all of gokus moves what he has done, is capable of in theory and ways he's dispatched enemies. Same as the vexs' capabilities in theory. Everyone is so quick to improv goku but act like the enemies can't alter their behaviors and attack strategies. They will still fight the same but goku has never not had morals of some sort. We know what he's capable of and we know what he won't do despite his power level. He wouldn't win because he's not a cut throat. Aside from bills and whis the only other I could see going head to head with an open willingness to destroy and go in swinging full out is kid buu. There's nothing to hypothesize on that. It's fact and just like destinys' bosses are coded a certain way gokus character fights and deals with new enemies the same way no matter who it is. Key word being "new". So returning villians he would naturally take more serious because he knows who they are and what they have done/are capable of and he expects them to be stronger. He will underestimate atheon and get teleported and get lost in time.