How can I prepare uneven concrete basement floor for vinyl planks?
Type of flooring dictates how "perfect" it needs to be, carpet being the least work. Vinyl is flexible so just no real drastic changes.

Below are the basic steps to fixing uneven concrete floors.

Use a straight edge and mark high and low spots. (Really exaggerated case in this image)


Or if you like gadgets, Bosch has a new laser out that lets you easily mark the uneven parts.


You use a grinder (with dust mask and safety goggles) for high spots. You can rent a large floor grinder or get on your hands and knees.


and SLC (self-leveling compound) in low spots. For SLC just follow the manufacturer's instructions. If the floor is really bad you may want to pour the stuff over the whole thing.