So LSSJ Vegeta would most likely toy with Seth for a bit and make him suffer as much as possible, via breaking limbs, using his body as a trampoline, swinging him around like a ragdoll, etcetera etcetera. Then once he's full he would just either rip Seth apart, crush him into a ball, or just use a Ki blast to disintegrate him. Oh, he'd also leave no witnesses by destroying the entire planet with ease and go on a multi-galactic or possibly even universal rampage :D
SETH just called me on Discord and told me that he recorded himself fixing the city he obliterated, and then posted the video to Imgflip on my account.