I always figured the bottom button thing to be a Saxon clothing convention. As per the OP, I don't typically wear one with my waistcoats. The exceptions being: when in a great kilt or philabeg, wearing it over a waistcoat to support a dirk (a rare thing limited to reenactments), and when wearing my Clergy tartan sportkilt as it has a low rise. The top of my waistcoats only cover about an inch and a half (if that) of the top of my kilt, so I think the belt looks okay with it.

That being said, my style is rather eccentric. So I wouldn't consider me an expert.