For cracks in wood flooring or substrate, use a wood filler material to cover them and prepare the surface. This not only helps to prevent those cracks from growing wider and causing problems with vinyl adhesion and placement, but it also helps to level the floor and clear up rough spots. You can look at your local hardware store for a common wood filler to complete the step.
If you are preparing concrete for vinyl flooring, switch to a patching compound instead of a wood filler. This compound is made of epoxy, a self-leveling, plastic-based material that you pour on the floor and let harden. This helps to block cracks and raise depressed areas. If your concrete floor has a lot of cracks, replace it entirely. Even patching compound will not stop an old concrete slab from cracking and eventually damaging your vinyl.
Backerboard is a material like particle board or fiberboard. Backerboard is made with cement materials, but there are other options as well. You can nail these boards onto plywood and other substrates and then install your vinyl on top. Backerboard is useful for covering minor flaws in your floor and solving gaps and other issues that are difficult to fix any other way.
If you are planning on a floating vinyl floor, this flooring will not be glued or nailed down. Floating floors are more forgiving of cracks than other types of flooring, so if you have only a couple of small cracks in an otherwise reliable surface, do not worry about patching the entire floor. However, you should still make sure that substrates are properly leveled and sealed before beginning.