But Vince, over on Jen's house's balcony, was topless. Jeez — for all I know he might've been completely butt-naked!
Literal
But, Jen [gen-の] house [gen-の] balcony [in-に] was Vince [topic-は] upper-body naked. Geez, perhaps stark-naked [copula-だった] [maybe-かもッ]!
A dense mimetic-laden description. 上半身裸 is the compound 'topless (upper body bare).' すっぽんぽん is a childish, mildly humorous mimetic for 'completely naked / butt-naked,' derived from ぽんぽん (for bouncing/bare surfaces). もしかしたら~かも 'maybe X / could be X' is the characteristic speculative frame. The trailing っ in かもッ represents a sharp sentence-final glottal stop, a spoken intensifier.