By that point he clearly surpasses Goku by a good margin & due to being the stronger fusee should have been the dominant personality.

Hell no. Goku did way more damage to Fused Zamasu who is far stronger than Black. Not to mention Goku always had Kaioken which made him superior than Vegeta.

Does fusion really work this way? I don't know if power levels really matter for this

Also, while Vegeta trained in the chamber and Goku not, Vegeta only did more damage to Black. Goku was almost beating Merged Zamasu alone and his kamehameha was stronger than both Trunks and Vegeta's attack.

IMO Vegito Blue should have stomp Zamasu more than how he did it in the show. Since in Buu saga both goku and Vegeta were useless against Buuhan. While in Super, they were still giving Zamasu (fused) a good fight!!

I don't think either is dominant. Look at the hypothetical fusion of goku and Satan. If it were determined by power levels Satan wouldn't even exist in the resulting fusion.

He is mostly vegeta in DBZ with a little of goku personality but DBS show his Goku side a little more.