SSJ4 Goku (post-LS) and SSJ4 Vegeta didn't look that strong compared to LSSJ4 Broly either, and they are superior to Nuova (vs SSJ4 Goku). I can tolerate your conclusion, but your reasoning is fishy.
On topic: Vegito's superior h2h combat would enable him to wear Buu down enough for him to finish Buu with an amplified attack, similar to what Goku would have done to Kid Buu if he didn't decide to give Vegeta a shot at him. Though, if Buu uses his Transfiguration Beam more effectively (e.g. turning Vegito into a liquid instead of a solid), then he could possibly take the win if he tags him with it.
Nappa (fresh and calm) vs Goku (post-Kaiōken attack on Nappa)
- Nappa can use Kamehameha with as much amplification as Goku's
- Goku can't use Kaiōken
- Nappa can't go Great Ape
- Win by death or K.O