@Mr_AW on Twitter displays the proper way to wear a Christmas vest (click pic to see it in its glorious fugliness):
Pairing it up with a very busy turtleneck is the preferred method, as shown here (bonus points for total lack of color/design coordination).
Alternatives for warmer climates are t-shirts or even better, a t-shirt with a dickey. “Don’ts” would include wearing a vest with a wifebeater, or with nothing at all under it.
At least add a tie or something.
(Thanks to @Mr_AW for “doin’ it rite”!)