The immensely powerful fusion of Goku and Vegeta using the Potara earrings.
Vegito, better known as Vegetto in the original version, is a mixture of both Goku and Vegeta's characteristics and would be their most powerful fusion if it ever reaches the Super Saiyan 4 stage. He is approximately 5 feet 8 inches tall, about the same height as Goku. His personality is a mix of the two Saiyans. Vegeta's cockiness shows in Vegito's personality, but it is tempered by his strategic mind and Goku's concern for others. (He ultimately used his cocky attitude to fool Buu into absorbing him, which gave him the chance he needed to save Buu's absorbed victims.) Being a whole new being, he refers to his counterparts as separate people as they would address each other.
Vegito's hair is more inclined toward Vegeta since it firmly stands upwards and is very dark brown in color. However, he has two bangs sticking out like a downward "V" shape, a trait caught from Goku. Vegito wears two earrings as result from the Potara fusion, and wears an inverted version of Goku's outfit. The inversion is caused by adopting Vegeta's outfit for the external wear, and using the orange of Goku's outfit for the undershirt. Finally, he wears white gloves and boots in the same style as Vegeta. He is very similar to Gogeta, who is another fusion of Goku and Vegeta.
The Potara Earring Fusion used to form Vegito is what Elder Kai considered to be the proper way to do fusion, as opposed to the Fusion Dance Technique. The resulting fusion lasts forever, even if the fused being removes the earrings. The determination of power levels is more complex than simply adding together the power levels of the two individuals, as it multiplies the users power levels together.. Vegito has the confidence and fighting smarts of Goku, along with the superior attitude and genius strategical mind of Vegeta. Goku and Vegeta's rivalry somehow made the fusion more compatible. Probably because each separate persona tried to give off as much power to the fusion as possible to show the other, giving them even greater power than expected, which was seen when Vegito transformed into "Super Vegito".
Vegito was a fusion formed out of desperation by Vegeta and Goku in an effort to stop Super Buu, an ancient and powerful being whose only purpose is to destroy. Because Buu had absorbed Gotenks and Piccolo, Goku and the Supreme Kai's came to a conclusion that the best hope of defeating Super Buu was fusion, but there was no time to attempt the Fusion Technique. As a solution to this problem, the Potara Earrings were given to Goku to save the universe from Super Buu. Goku then proceeds to Earth just in time, before Super Buu could destroy it. Goku threw the earring to Gohan so they could fuse, but Gohan failed to catch it. Super Buu then lost a large amount of power (over one third by Goku's observation) due to Gotenks at separating inside him. He then proceeds to absorb Gohan, becoming even deadlier and more powerful. Goku is now left with no one that could help him by fusing, but miraculously he senses Vegeta's energy, and quickly transports himself to his location. Super Buu suddenly comes out of no where and Vegeta fuses with Goku. In the anime, Vegeta initially ignores Goku's plea to fuse with him and they both send ki blasts towards Super Buu. Super Buu easily reflects their attacks with his own ki blast. Super Buu then taunts them and Vegeta attacks Super Buu but he quickly beats them both up. After some quick persuasion, Vegeta finally agrees (after being told what fate had befallen his family). Then Goku tells him for the first time that the fusion would be permanent and Vegeta shouts at him telling him that he should have told him that in the first place. They both then wear the earrings giving birth to Vegito.
Once Vegito is formed, he tests out his new body. He then proves himself more than a match for Super Buu as he easily withstands an assault that would have almost killed the others separately. Vegito then overpowers Super Buu in the ensuing battle, clearly showing that, even in his base form, the fused Saiyan is much more powerful than the mass-absorbed Super Buu. After proving his superiority to Super Buu, Vegito then decided to turn things up a notch, by increasing his power by transforming into a Super Saiyan, and calling himself "Super Vegito". Super Buu is impressed, but not convinced that Vegito is stronger than him. In the manga, however, he turns Super Saiyan immediately upon fusion.
Vegito then begins to completely dominate Super Buu, rendering his attacks useless. Super Buu then liquefies himself and goes into Vegito's body through his mouth. Once Super Buu is inside, he makes Vegito's muscles bulge tremendously and tries to scare him by saying he is in control of his body. Vegito then uses his energy to isolate Super Buu inside his body and beats him out. Super Buu becomes enraged because Vegito is making fun of him. His anger at this caused him to start breaking through dimensions, which, if left unchecked, could have destroyed the universe. Eventually, Vegito forced his way through Super Buu's shield and punched him in the face, which stopped his attack.
With little left to lose, Super Buu turned Vegito into a coffee flavored jawbreaker. All seemed lost until Candy-Vegito revealed that he was able to fly around and still attack physically beating Super Buu in comical fashion, eventually slicing off Super Buu's antenna. Super Buu eventually turned him back after realizing that there was no way to hurt Vegito because of his small size (despite the fact he couldn't hit him before anyway). Super Buu then returns Vegito back to normal.
After another beating, even Super Buu's regeneration was starting to fail him. Vegito decided that it was time to end the battle once and for all. He then gave Super Buu ten seconds to prepare for death, and told him to do whatever he feels like in that time period. Super Buu started to panic, and when Vegito reached nine, he manipulated the antenna that was recently cut off and engulfed Vegito. The battle was over with Super Buu coming out as the victor.
Unknown to Super Buu, however, Vegito could've easily destroyed him at any given moment, yet he toyed around with him. Before being absorbed, he had set up a barrier, so he wouldn't be integrated into, as Vegeta called it, "Buu Goo." This is because he wanted to be absorbed so as to free all his companions and family members. Thus the reason for cutting off the antenna and giving him the free time. Super Buu then goes on a rampage, and Vegito is left within him to free everyone. When he entered Super Buu's body, the magic of the earrings wore off because the stomach acid dissolved the fusion, and Vegito split into Goku and Vegeta again. Vegeta then crushed his earring so they wouldn't have to fuse again. When facing Kid Buu, they were also given the option to become Vegito again, but they refused out of Saiyan pride.
This is Vegito's base form. This form is first seen when he is first fused. Even in his regular form, he is immensely powerful. At this stage, Vegito's power and speed are far greater than a Super Saiyan 3's. He is even much more powerful than Super Buu w/Gohan in this form. Additionally, Elder Kai stated that Goku and Vegeta's rivalry made their fusion more complete, giving Vegito even more power. Therefore, Vegito would still be able to dominate Super Buu even if he were to sustain this form, as it was shown that after a short fight, Vegito proved more than a worthy opponent for Super Buu, overpowering and outclassing him in every aspect. Even in his base form, Vegito is still the most powerful character in the whole of Dragon Ball, Dragon Ball Z and the Dragon Ball manga. However, he is able to make a transformation and substantially increase his power and abilities. Also note that in the manga, Vegito directly transformed into a Super Saiyan, calling himself "Super Vegito".
After being turned into a coffee-flavored jawbreaker by Super Buu so that he can be consumed, Vegito is still able to fight as a small candy. Some consider this to be (ironically) Vegito's most dangerous form. Not only does Vegito retain all of his power, but he is also extremely fast and far too small for Super Buu to hit; thus using an original attack to shoot in Super Buu's mouth and out the back of his throat, unscathed. In the dub he calls it his "Pinball Attack". However, while this small size gives him a colossal advantage in battle, he is not invulnerable. Vegito is unable to use any special abilities in this form, rendering him unable to kill villains like Cell and Buu as long as they can regenerate fast enough, although they could theoretically be beaten to death or shot full of holes due to Vegito's speed and size. This is however, a situational case as no one else possesses it.