Vegetto is formed during the Buu Saga. The fusion takes place when Gokou and Vegeta get permission from God to return to the Earth to fight Buu.

Gokou tried to give Gohan one of the two Kaio-shin's earings, but buu gets to him first. Instead he ends up giving it to Vegeta, and tells Vegeta to put it on his ear. Directly after this happens the two fuse into Vegetto.

Facts:

1) In this Fusion the two people that are going to fuse do not have to be at the same power level to fuse.
2) The fusion using the earings is Permenant.