If you have an uneven subfloor, there are a few ways that you can level it out before installing your vinyl plank flooring. The first is to use a self-levelling compound. This is a pourable mixture that will smooth out imperfections on the surface. Secondly, use thin sheets of plywood or OSB  to create a level surface. You can screw or nail these down to the subfloor and then install your vinyl plank flooring on top. Finally, you could also use shims to level out the subfloor. This is a more time-consuming option, but it will get the job done. Place shims under the low spots and then tap them into place with a hammer.