Sometimes you have to make the best of what is there. A very cost effective way is to use a product like Level Quick self levelling cement. The use of this product should be done as efficiently as possible to ensure a smooth seamless surface.
The use of a primer adhesive is also recommended between plywood and or concrete substrates.
The misconception of the self level compound is that it will level out completely with out the use of man power and or tools.
Flooring Installers know this is not the case, and when matching a high spot and a low spot in a room should be manipulated to not build up the high spot anymore, and to not put all the concrete in the low spot. The product is fairly expensive and should be used where needed. This pour is to make a smooth surface for the vinyl Plank floor to rest on. The floor was un even with high spots low spots. As you can see form the photo the surface is smooth and ready for the floor covering of Your choice.