Buttoned all the way out last button left undone?
Out = or. Fecking auto correct.
The button's there for a reason.
If it has 3 buttons, it's your call. If it has 4 or more, leave the bottom one undone. Or do it up. It's your call, really. Generally though, just don't wear a waistcoat unless you're in a wedding party :/
I hope to god your waistcoat doesn't look like the one in the pic. If it does, get that shit slimmed.