Prove me wrong. I'm a lore expert on Destiny, so odds are I can refute any argument you provide that favors Destiny. Vegeta and Goku would beat the entire Destiny universe in a fight.
Lets also assume they know the mechanics for the VoG and Crota's End.
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Pretty sure this has been mentioned, but Vex could just go back in time and kill him. Sorbet did it with a ring while SSJgodSSJ, vex can do it to baby Goku and Vegeta. Plus instant transmission and instant information sharing would make it impossible for then the kill the Vex before then