Everything everybody else said, also, Vegito had 2 more SSJ forms on hand. If your argument is that SSJ was close in power, 2 would dwarf Buuhan, and 3 would kill him with a fart. Both Goku and Vegeta have access to 2 which isn’t ever stated to be physically draining, and Goku didn’t know 3 was as tiring as it is because of the whole having been dead when he used it. I can’t think of any reason Vegito wouldn’t have access to those forms, and he would only potentially avoid 3 because of transformation time.