Is It A Rule If No One Knows About It?

I've heard that it is a "rule" of menswear that a double-breasted jacket should ALWAYS remained buttoned. Unfortunately no one seems to have told some of New York's most stylish men. Personally I don't think it is a big deal but some guys get really worked up about it. Does wearing a vest make the rule null and void or is it a worse offense to wear a vest with a DB jacket?