Huh? You ask. Okay, maybe not. Most people have at least heard of the cummerbund, that pleated broad silk or satin belt-thingy worn with a tuxedo. But "waistcoat?" Not to worry: it's just another word for "vest."
Okay. Now we have our terms straight. Is one better than another? Nope. Is one more formal than another? Not really. Traditionally, it's a matter of timing. Tuxedos (with cummerbunds) are worn after six. Morning suits (with vest/waistcoats) are worn (surprise!) in the morning. (And you do remember that the pleats of a cummerbund face UP, the better to catch crumbs, say some, or -- this one came as a surprise to me -- to hold theater tickets!)