They begin 30 feet away from each other
Vegeta is at his peak
Battle takes place at Cell Games Arena
Mentally, Despero controls Vegeta.
If they fought without mind control, Vegeta certainly has more then enough power to put Despero down.
And stop using solo'd the JLA case. Black Adam has done it, Deathstroke, Amazo, Doomsday ect ect ect
Someone who planets busts with a percent of his power can easily take this.
Flash doesn't have any enemies equal to his power that's why Zoom was created. Flash has to be dumbed down when he is in the JLA otherwise there would hardly be any bad guys to fight. And moving hundreds of times faster than light is a pretty significant superpower.