If you intend to go around with your suspenders visible, well, then I suppose suspenders which are compatible with the rest of your attire in terms of formality, color, pattern, fabric, etc., would work best. (Note that "compatible" isn't necessarily the same thing as "matching." It's much the same way as how a pocket square, necktie, and suit, when worn together, should all be compatible with each other, but it's typically best if they do not all match each other.)
2. A vest should properly cover the waistband of one's pants. A belt will not infrequently tend to protrude under the vest, adding girth, drawing the eye to the waist, and ruining the smooth line. Consequently, if you're wearing a vest, suspenders may be an especially good idea.
3. Assuming your pants have suspender buttons sewn in (please do not wear clip-on suspenders), you may wear suspenders with them, even if the pants also have belt loops. This is generally acceptable (although some individuals may not care for the look). HOWEVER, see #4, below.
4. Technically, pants meant to be worn with a belt, and pants meant to be worn with suspenders, are tailored slightly differently. So, if your pants have belt loops and you routinely wear them with a belt, you may find that they won't hang quite as well when worn with suspenders as would a pair tailored specifically with suspenders in mind (and these pants specifically intended to be worn with suspenders would, presumably, not have belt loops). This may not necessarily be a major problem - a given pair of pants may prove acceptable when worn either way - but it is something to keep in mind. Certainly, if you ever went to a reputable bespoke tailor to have a suit made, he would want to know your preference, re belt/suspenders, and would adjust the fit accordingly.
BTW, bonus points to you for calling them "pants" and "suspenders," as opposed to "trousers" and "braces."