Vegeta knew he was going into a death fight. He thought he could take Boo with him. It wasn't until he started to fight that he realized Boo was otherworldly and couldn't be brought down by conventional methods.
This is shit Gotenks should also know since the kids watched Vegeta's fight with Boo, and knew that the self-destruction didn't work (obviously). Gotenks's confidence in himself makes no damn sense, just like Kibitoshit thinking he could help in the battle against motherfucking Gotenks-Boo. Hell, they felt Super Saiyan
3 Goku fighting Boo and still had the gall to think they could win. Get that weak shit outta here, broskeet.
Base Gotenks (pre) > SSj3 Goku
:mikey