Which buttons are supposed to be undone on suits and vests? Well, it's really more of a personal touch than an actual rule- but, there are standards that you should abide by.
On a suit- never, ever, EVER, button the bottom button. You might see people do this on television- but it is dead wrong. In no way shape or form should you button the bottom button on a jacket.
Just follow this chart:
Essentially, 3-piece suit you should button all the buttons but the last one. If you are doing a business casual look (sport coat...) then some people will leave the top and the bottom undone. If it is more casual with a pair of jean, you may only end up buttoning a couple of the buttons while a few of them open.
All the above scenarios are more driven by personal flavor, the only one that I would make sure to abide by is the vest with a suit- button them all except the last one.