Borderline.. You dont really want your waistband or worse, a belt, peeking out. And definitely not your shirt or the bottom of your tie.
If you're going to wear a three piece, the trousers need to have a high enough rise for the top not to be visible under the waistcoat. If you're wearing the three piece, braces would also be the norm. If you can make out the bottom of the waistband it's no big deal, but the main thing is for there to be a consistent smooth transition from the waistcoat to the trousers.
Personally I have all my suits made as three pieces, and all my suit trousers with a higher rise. But with less formal suits, e.g. linen, I also put belt loops on the trousers and sometimes wear a belt and leave the waistcoat if it's very hot. Each to his own. Just don't let anything obvious like a belt of a glimpse of shirt peek out of your waistcoat - it will make it look too small even if it isn't.