Vegito is a lot like Vegeta more than he is Goku when he fought buu he was cocky and would often toy with him Same thing with zamasu he was cocky and toyed with him like when he pierced Zamasu with his spirit sword he said " Oh I'm sorry is this hurting you?" and he also said, " aren't you invincible". So in conclusion their personalities are a little bit similar.
first of all, Vegito toyed with Buu because he was trying to free the others, as for Zamasu he made some jokes here and there, but he was going for the kill pretty early.
