I'm going to show you what styles you should have on your waistcoat. Bespoke and semi-bespoke tailoring means you will have more say about this when communicating your wishes to your clothier/tailor. But with off the peg you have very little choice but the current fashion.
Single breasted or Double breasted?
For my money I would invest mostly in single breasted and have one double breasted for special occasions like weddings or casual. Double breasted looks great as a casual look rather then a work one.
So with single breasted waistcoats I would do a 5 or 6 button front. The 6th button may hang on the angle of the point on the bottom of the waistcoat on some models rather then in line with the other buttons but that depends on your tailors designs. Let them know if you prefer one and see why hey can do.
Traditionally you should have the bottom button undone on a waistcoat at all times. This may date back to needing to undo it for riding purposes on horseback. There is a story that the Prince Regent couldn't do it up, cause of his mid-riff size, and it became a fad that stuck. Personally, it isn't the end of the world either way, just do what is comfortable to wear as both versions look great.
With single breasted you will also have points on the bottom edge whereas with double breasted you will always have flat waistcoat bottom. The below models are the common and safe versions of both.
These are straightforward models and each have a set of lower pockets. If you are tailoring then you should be able to choose, or be advised to, position of top button position, length of waistcoat, how trim it will be around chest and waist, and so on. With off the peg you will have what is available in the sizes.
For me a good piece of flair for a waistcoat is a lapel. It adds a great twist to an already superbly dressed look and is a must for at least a third of your waistcoats. There are no events that will shun lapels on waistcoats but do lend themselves to casual very well and tweeds especially.