Well, if I were to apply some actual logic to things, we could assume that Vegetto charging and concentrating his Ki into the Final Kamehameha was akin to him concentrating all his SSB power temporarily like Galu is now doing permanently. Perhaps the theory of Vegetto only surpassing Beerus with that Final Kamehameha is true and that Full Power SSB Vegetto > Beerus >> SSB Vegetto >>/> MSSB Goku > Merged Zamasu.
Either that, or you could assume Vegetto got plot haxed like in GT, given his performance against Zamasu was quite similar to what it was against Boo.