Evil Vegito is a clone created by Ukele using the Ukele Dragon Balls to fight and kill the Z Fighters during his quest (The Ukele Saga).
Evil Vegito is identical in power to Vegito, the Potara fusion between Vegeta and Goku formed to fight Super Buu. Evil Vegito even has his own pair of Potara ear rings that function as real ones. His moveset is the same as Vegito's, but he prefers the technique known as "Final Kamehmeha" as it is quick to charge and very powerful. Also, if he charges it for longer, the move is more powerful. Saying the name of then ame isn' required, though saying it while charging appears to give the attack more strength.
Evil Vegito is summoned on Namek by Ukele and immediately told to hide his power level, as the Z Fighters were still on Earth in the middle of a battle. Ukele planed to use Vegito to attack and kill the Z Fighters after regaining the Ukele Dragon Balls, so that he might use them in a time of need. Due to the wording of the wish, this Vegito clone was under Ukele's control, whom was so misguided that he commanded the clone to do evil things, even though Ukele believed his path to be a righteous one.
The Ukele Dragon Balls, due to being created in a non-true way, are fragile and seem to have some drawbacks to their power. One such draw back is that it appears as though Ukele's control over this Vegito clone weakens with each other clone he summons. After summoning clones of Cell and Frieza, it is apparent that Vegito's individuality is returning. This is apparent when Ukele requests Evil Vegito to give him the Potaras, and Vegito is hesitant and not pleased with such an action. However, Ukele doesn't seem to notice and is surprised when Evil Vegito eventually turns good, thinking it is some Saiyan power he did not know about.
In reality, after forcing Cell and Frieza to fuse into Cellza, Ukele left for Namek and used the still gathered Ukele Dragon Balls to make 3 more wishes, creatig 3 more clones under his controll. After this, it was shown that Evil Vegito was almost completely free of Ukele's control. Upon Ukele's return, and his ordering of Buu to attack innocent people, Derig decided to turn on Ukele, who he now saw as an evil Namekian. Using what he dedeuced about Vegito and the fleeting power Ukele had over him, he volunteered to go with on Ukele's order for Vegito to search out the Z Fighters.
However, Derig was able to start Vegito's struggle against what remained of Ukele's control, and with the help of Goku and Gohan's encouragement, eventually broke free, but not before knocking Derig unconcsious during his violent fit of struggle.
Vegito then helped Goku and the Z Fighters fight Ukele and his minions instead of killing Goku or Vegeta before the two of them could fuse into Gogeta.
During the fight, his seeking revenge on Ukele was stalled by the ppower of Cellza and Coolemba, and Buu absorbing Cellza made an even match against SS3 Gogeta and Vegito.
It was during this fight that Gogeta suggested that the two of them fuse before Gogeta's fusion timed out. However, Ukele was able to destroy the Potara and the Z Fighters once again were at the mercy of Ukele. However, due to Buu's absorption of Coolemba, Ukele barely regained any control, and the Z Fighters were able to escape after Gohan's death.
Due to the combined efforts of Derig, Bulma, Vegeta, Goku, and Vegito, the rest of the Dragon Balls were recovered in a short time while Buu rampaged across the world. With the remaining Senzu beans, they set off to confront and defeat Ukele and Buu (Final Form) using a plan devised by Derig and the sheer strength of the new fusion that was possible due to the wish being granted for a new set of Potara ear rings (Black Potaras) that allowed defusion. However, before the second wish could be granted, Buu attacked as Derig left to enact the plan against Ukele.
Goku and Vegeta performed the fusion dance to form Gogeta, and Vegito and Gogeta fused to form Gogito while Master Roshi distracted Buu for a few seconds.
Gogito and Buu's fight destroyed much of the Earth and covered great distances before Gogito took the fight nto space, where he went Super Saiyan 3 and performed a 100x Final Big Bang Kamehameha, lanching Buu into the sun, where he was presumedly killed.
Vegito then remained with the Z Fighters in this timeline as a part of their team, and they kept the Black Potaras for future use.
Evil Vegito's appearance differs from regular Vegito by the clone (Evil Vegito) having slightly darker colored clothes and darker colored eyes. Also, when this Vegito charges, black lightning bolts appear around him when in Super Saiyan, but not when powered down.