Vests and waistcoats are most graceful garments that add more to an outfit than most accessories. They can add procedure to suits, style to unmatched outfits, comfort to casual wear, or warmth to business dress in cool weather.
A three-piece suit is an excellent starting place for a man with no other vested outfits, as the vest can be worn separately with other garments - the only care here is that any piece of a suit will, if worn too frequently, wear and become lighter in color than the other pieces. Rotate the wardrobe to keep everything matching properly.
After a first waistcoat, the decision of how often to be dressed in vested garments is really every man's choice. Any style worn without difference gets boring, so don't be "the vest guy" - but have at least a few standing by for days when a little extra flair is called for. The versatility and unusual style of waistcoats and vests is too good for a well-dressed man to pass on entirely.