I was thinking that there was a convention with 6 buttons, of leaving the bottom button unbuttoned. If the vest is long, this might make it lie better while seated. Proceeding to rummage through my closet, I find mostly 5 buttons per vest.

The oldest in my possesion, from a circa-1970 suit, has 6 buttons---and the lowest is completely non-functional, being on the tapered edge near the point, and having no corresponding button-hole! If i'd ever noticed this before, I certainly had forgotten.