That said, a single vent still looks really good when cut well. It needs to be kept longer than you find on most ready to wear and cut with a little spring to it so it has a little movement and doesn't splay apart when you stand.
Double vents are considered a little more formal. Single vents come loosely from riding wear, where the single vent allows you to ride a horse without the jacket riding up etc. So it has a slightly country and casual connotation. This can look good when teamed up with a cloth that is more formal as you get the interplay between the two styles.