User blog comment:Cryuga/Note to self/@comment-2207258-20120415045537/@comment-1739368-20120709170542

You would also have to assume his stand has the same effect as the actual sun. He used it to increase the temperature, but Vampires being weak to sunlight is more of a mythical thing rather than something logical, so there isn't really a way to prove that it would have the same power.

Considering the size of it, I doubt it would fit in a building anyways. Even if he could shrink it, he has to be nearby, and without some type of camoflauge like he used in the desert, Dio pretty much screws him over with time stop.

We didn't learn it until part 6, but Pucci was also with Dio for a majority of the time before he confronted Jotaro. Pucci's stand removing powers would proubably ensure his safety.

I'd like to think one of the main reason was that he really was stupid though.