If you don't own a suit, I think you dodged a bullet for the time being, even though it would definitely be better to have one, because the jackets are generally nicer. The rule of thumb is you should dress the same as for an in-person interview, but, of course, if you position the camera correctly, you could get away with no pants at all, so you don't "need need" a suit if you don't already own one and have a blazer that is nice enough to pass for a suit jacket.
I have a very hideous black-ish blazer made out of a weird-looking, almost corduroy material with thin green accents running down the jacket longitudinally. It is a menace. I wore it to a virtual interview recently and it simply looked like a black jacket, presumably as a suit piece. Interview went fine, I looked fine, and I was accepted to the program. I agree with KnightDoc that virtual interviews have given us some liberties when it comes to the capriciousness of professionalism. If it looks normal on camera, you should be fine.
A friend of mine works in higher ed and while he was interviewing for jobs, the director asked him to stand up for a “group exercise” right when they started the interview.
