People wear belts with vests in many styles of clothing, so it's not that you should not wear a belt. The issue is the HUGE buckle.

The "traditional" waist plate style belt buckle that we see on most kilt belts is designed to be seen. Unless you're wearing a mid-riff vest so that it does not obscure the buckle wearing a buckle with the vest will be pointless and look too busy. The other issue is that a huge shiny buckle surrounded by all the shiny buttons on the coat and vest in conjunction with the shiny cantle on the sporran and probably a shiny kilt pin will be too much bling, kind of like a pimp peacock .

"Too Busy", Cluttered or gaudy is the number one sin that people commit when wearing the kilt. People tend to mix-n-match so many styles and formality levels that an outfit quickly becomes horrible to look at. K.I.S.S. (Keep It Simple Scotty) If in doubt, leave it out.

Black tie is black tie. It will look fine. Just ensure that the tie does not reach past the bottom of the vest. That would look very sloppy.