A waistcoat alone is not ideal, but if that's what you have on hand, than wear it proudly. It's a not a full PC, but it will sure look a heck of a lot nicer that the guys in dress shirt and dockers. Please make sure that you are wearing good kilt hose and nice shoes as it will finish up the outfit. I have seen picks of people that are dressed up top, but wearing pushed down hose and boots below or black runners.

There is a Simplicity pattern that has an argyle type jacket on it, but I can't remember the number right now