From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
Vegetto puts both of his hands forward and combines the massive energy of the Final Flash and the Super Kamehameha techniques into a medium-sized, whitish-blue energy sphere. He then fires the energy sphere at his opponent, inflicting an enormous amount of damage and leaving a huge cloud of smoke in its wake.
Dragon Ball Z
Vegetto used this attack in his battle against Super Boo (Goten, Trunks, Piccolo, and Gohan absorbed). The Final Kamehameha was powerful enough to destroy Super Boo (Goten, Trunks, Piccolo, and Gohan absorbed)'s left arm, part of his antenna, and the bottom half of his body. Super Boo (Goten, Trunks, Piccolo, and Gohan absorbed) managed to regenerate from the damage he received, however, and continue the fight against Vegetto.
Final Kamehameha appears as one of Vegito's techniques in Dragon Ball Z: Hyper Dimension, Dragon Ball GT: Final Bout, Dragon Ball Z: Shin Budokai, Dragon Ball Z: Infinite World, Dragon Ball Z: Budokai Tenkaichi 3, and Dragon Ball: Raging Blast. In Budokai Tenkaichi 3 and Raging Blast, it is his Ultimate Blast in his Super Saiyan form. In the games, he holds it out like a Final Flash and moves it back like a Kamehameha, and then releases it in a golden-yellow colored beam (in Raging Blast, it is a blue and white color). It is interesting to note that, while the attack is available to Vegito's Super Saiyan form only, he never used it in the anime as a Super Saiyan.
When performing the attack in Dragon Ball GT: Final Bout, he yells, "O tsurugi wa koitsu da! FINAL KAMEHAMEHA!". This has never been seen or heard before in the anime or manga at all. Also, in the games Dragon Ball Z: Shin Budokai and Budokai Tenkaichi 3, he simply says, "Final Kamehameha!" when this attack is performed.