To make a waistcoat/vest, you'll need about two yards of solid-colored 45" wide fabric, if you plan to make the back from the same fabric as the front. If you're wanting to amke a tartan vest, with the tartan cut on the bias, it will eat up more fabric. I'd say 3 yards should do it, with a good bit of scrap left over.

I can't help you on the weight of the fabric. You might be able to ask the company you got it from.