Adding a vest to a suit makes for a great look. The vest, also called a waistcoat, has always been a part of good menswear, and even though it may be a bit more popular now than it has in recent years, I still very seldom see a three piece suit. It's a shame because it's not only complimentary to a suit, but it looks great without the jacket. In fact, I recently had our tailors make me a trouser/vest combo and didn't have the jacket made. While I'll be adding the third piece sometime soon, I've enjoyed wearing the trousers and vest during the summer when a jacket can be too much because of the heat. A lot of guys find themselves removing their jacket throughout the day for better mobility at the desk or to avoid getting too warm. Unfortunately they lose the flattering lines a suit jacket provides. The shirt balloons and the tie flails about, and what used to be neat and tidy becomes sloppy. The vest restores the flattering vertical lines and the V shape on the torso, and it keeps the shirt and tie tucked and trim. Next time you get a suit (or preferably, have one made) consider adding a vest. You'll add functionality and style, and it's a great alternative if you're not wearing the jacket full time.
The vest should fit well for optimal effect, and the best fitting vests are always the made-to-measure variety. A vest should be snug through the midsection and chest, so off-the-rack vests frequently don't fit correctly as, like jackets, they're made to accommodate a wide variety of people but not necessarily fit them well.
A lot of guys may not feel entirely comfortable in a vest without the jacket. The back on many vests today are a silky material that some may not want exposed because it's shiny and looks a bit fancy. Traditionally the vest back was made of the same material (wool) as the front. That's largely been done away with because it's assumed the vest back will seldom be seen, and it's cheaper to use less wool. It's still possible to get a vest with self material on the back if you go custom. I prefer mine to be made with the same material all around. If you're getting a vest made by a tailor, take advantage of this option and get it the same all around. It's a mark of a custom-made vest.
A vest should ideally be one to three inches higher than the top button of the suit, but not much more than that. Generally speaking, a three button suit goes well with a five button vest. A two button suit does well with a vest with three or four buttons. The point is that because a two button suit has a deeper V than a three button suit, the vest should follow suit (no pun intended) and correspondingly be a bit lower.
The vest can be single or double breasted (yes, double breasted works with single breasted suits). It can have lapels or no lapels. If you plan to regularly wear the vest sans jacket, lapels are a good option since they add a bit of what's lost by being jacket-less. The length should be such that shirt material isn't exposed in front or in back. I've realized through personal experience that a vest should be requested to be a bit longer if you wear your pants on your hips instead of around your waist like I do. I have a vest that's a bit short in back as a result and it's a hassle making sure no shirt peeks out the back.