Vegetto is formed when Goku and Vegeta form using the Fusion Earings. The earings where designed so that when put on any one can fuse together but the process is permanet. Instead of the original. He arrives to find that Majin Buu is even more powerful. Already aborbed Piccolo,Gohan,and Gotenks. Vegetto is stronger then any other fused saiyon.
Vegetto ia very powerful he could have easily defeated Buu but had other plans. He went through one of Buus pores and retrived his friends including Fat Buu who was good. Making Buu go back to his orignal state. But during this process became unfused.