Having been properly trained in the the wearing of vests/wasitcoats from a young age, I never button the bottom button of a vest unless it's the 3-4 button formal version or a double breasted model. It doesn't matter whether I'm kilted or trousered.

If you do--or do not--no one will care. It's not that big a deal--and quite frankly, I never thought of it before--but since I was taught not to button (and I've noticed many well-dressed men unbuttoned), I don't button.

And it obviously looks good buttoned, witness the good Duke of Rothesay's photo provided by Joseph McLaren above. One thing about ole Charles, he looks pretty snazzy in his clothes.