It was an old rule that nobody should ever see the back of your vest. That's why they were often made of a different material. They were only worn with a jacket over. In that case the vest would be closed with the exception of the lowest button and the jacket (single breasted) open over top when sitting, and perhaps one button done (near center).
Wear it how you like, but you might as well know when you're breaking traditional rules (which I'm not suggesting you don't do, but do it knowingly).
For men, it is imperative that the vest does not always have to match the dress pants. But it must correlate with the dress shirt. With patterns like dots, a black vest vertical striped complements it along with a grey dress pants. No tie is needed but the dress shirt should be button up except the last button near the neck, only button that one when you have a tie.