Regarding Buu and Fusions.

[Vernichter]

If Vegetto was split into two because of the way Buu's body works, why wasn't Gotenks automatically returned to being two people when he was absorbed?

I guess you could argue Fusion vs Potara, but I think this was just a bad plot device to split Goku and Vegeta.

[Bussani]

Because Vegetto was an intruder, like a germ, while Gotenks was purposely absorbed and wrapped up to steal strength from. Buu wouldn't want Gotenks to be split up, and he seems to have good control over what his body does.

[Kaboom]

I always used to assume that Vegetto's force-field somehow interrupted the natural absorption process and resulted in him being "broken down" for digestion instead.

Or it might be some sort of incompatibility issue with Majin Boo being crossed with Kaioshin-magic.

[Herms]

In the series Goku speculates that Fusion still works inside of Boo's body while Potara simply doesn't due to the "bad air", and he proposes that he and Vegeta perform Fusion so they can defeat Boo. Pretty nonsensical, but there it is.

[Son Edo]

1 Gotenks was successfully absorbed, Vegetto wasn't. 2 Fusion you eventually split apart, Potara doesn't. The air alows Gotenks to stay fused because he will eventually split, Vegetto splits immediately because he's fused forever. Maybe that could be it?