It all depends on the type of vest. There are waistcoats that are formal and others more casual. Some are utilitarian and winter outerwear.
Formal:
http://www.josbank.com/IWCatProductP...duct_Id=417466
not:
http://www.campingsurvival.com/khakdelsafou.html
casual:
http://zemsky.stores.yahoo.net/wrdeve.html