Waistcoat/vest sizing?

I am looking at some waistcoats/vests to possibly wear with an odd jacket or sport coat. It occurs to me that I have no idea as to how to size these.

I am not really looking for a sweater vest, but rather one that buttons in the front.

Can anyone offer some help as to how to size these properly?
Does one go by waist measurement? Suit jacket size? Something else?

Thank you for any time and help!

Waistcoat/vest sizing?