Need help with preparing concrete floor for vinyl?

There is a floor leveling compound that seeks its flat surface. But, you have to find someone who knows how to use it. May be one of the more reputable big flooring companies. The reason why I am saying this is because we used a local flooring company (small town) to install some vinyl for us and the floor had a couple of spots that were low. They didn't want to do it, but we insisted as a condition of the job. When we checked on them after work, we found a huge lump of leveling compound in the middle of the floor. They had to come back and chip it out and clean up their mess. Don't know what their problem was, but it was a lesson in choosing contractors more carefully.

Floor is not smooth but has has tile laid there before. Do I need to add a kind of thin board so it will lay smoothly?