Vejiita-Baby...no, that isn't just a cute little nickname I made up for my favorite Dragonball male. Vejiita-Baby (or Vegeta-Baby) is the correct way to reffer to Vejiita, completely taken over by Baby. He becomes Perfect once he has possesed everyone on earth with a few exceptions. He calls upon all the Saiyajin's to give them their power.
The Saiyajin's, who are Goten, Gohan, Trunks, and Bra (as Gokou and Pan are safe) give him their power, though I won't say if they do it willingly or not.>Once Baby has gained the Sayiajin's power he grows buffer and gets yellow and red fin-like extentions off his body. Then he calls upon all the Tsufuru-jin that he has taken over to lend him their power. They, choicelessly do.
Baby takes their powers and becomes buffer, and more punk-like. His hair goes all white, and his clothes are black, with red and yellow Freeza style jacket.
During all this, Gokou is watching. Vejiita-Baby is undertaking this transformation to show that he is more powerful than Gokou. He wants to show off all his power before killing and destroying Gokou. Once powered up, he plays around with Gokou for awhile, then creates a giant black shere.
The Sphere is called The Revenge Death Ball. Vejiita-Baby is using it against Gokou because Gokou almost killed him orignally when he was only a Baby. Vejiita-Baby hurls the Revenge Death Ball at Gokou, and Gokou just stands there while it decends on him, impressed with Vejiita-Baby's power.
I won't say how Gokou survives the Revenge Death Ball, but he does. He is taken to Kaiou-bit and Kaiou-dai's place where he has to undergo special training to become powerful enough to defeat Perfect Vejiita-Baby. This training, requires the regrowth of his tail, so he can access all of his Sayiajin powers.