Originally posted by StiltmanFTW
Fusion Reborn, which most likely is canon to GT, had stronger Goku fusing with weaker Vegeta, I guess Goku simply lowered his ki.
Both Goku and Vegeta were in base when they fused in that movie
I just assume they have rough parity when they have the same forms, unless explicitly stated, like Vegeta needing the Majin boost to be even with SSJ2 Goku
Originally posted by AuraAngelSSJ4 isn't a static multiplier. It brings the user to their utmost limits--in that regard, it's similar to Elder Kaioshin's ritual. In Goku's case, the SSJ4 ritual just happened to be at least 10x SSJ3.
Base Goku and Vegeta have to be around the same level of power in order for the Fusion to work. It wouldn't make sense if Vegeta was THAT badly outclassed by Goku in just his base form. SSJ4 is just a multiplier, not an immediate equalizer.
I guess my point is that since Vegeta was demonstrably weaker than Goku in base/SSJ/SSJ2(see his battle with Super 17), and with SSJ4 he matched a powered-up SSJ4 Goku, it would mean Vegeta got a much larger boost from SSJ4 than Goku... Because you're right: SSJ4 Goku=SSJ4 Vegeta--that's why they were able to use Metamoran fusion without adjusting their ki beforehand.