Painting the siding a color that's darker than the original color may cause the vinyl to absorbe more heat than it was designed to (because of the darker color.) Thus, you'll have more expansion and contraction, and after a while, the paint job may look terrible. First, clean a small INCONSPICIOUS area and just paint it (after the wall is dry.) Be SURE to follow the Mfgr.'s Instructions on the can like don't paint in direct sun and wait 2 days before you do a 2nd coat, and so on. Understand, vinyl does 'chalk' a bit so be sure you clean the vinyl with a Vinyl Cleaner. (It's like $5 per gallon at Home Depot ...)
Wharping is more probable with the darker colors because they absorbe more sun/heat/UV. (The lighter colors 'reflect' very well, on the other hand.) With vinyl, especially, when it comes to expanding and contracting, the darker colors will cause that. Vinyl is a very thin product, and if the heat isn't adequately dissipated very well on the UNDERSIDE, you will have a relatively higher degree of expansion and contraction (afterwards). You may have to prime, but you won't know for sure unless you try it out. Obviously, after cleaning and painting, if you find the paint isn't adhering, then eithe a better cleaning job needs to be done or you need to prime.
Have you thought about JUST cleaning the siding? I mean, you have to do that anyways if you want your primer to stick. My home happens to be 100% white vinyl sided. You could think about 'accenting' the house with different colored shutters and trim. THIS may be cheaper than painting or even re-siding the home. Use 100% Acrylic House Paint. Oh, I personally wouldn't recommend painting vinyl siding. 1) A lot of times, it looks horrible, 2) in a few years, it may start peeling, and 3) you've basically turned a maintenance-free siding material into one that requires maintenance.