Well there about one hundred quad trillion floor levelers ranging from crappy to very good. Some are self leveling other have to be trowled. Some have an allowable thickness of one inch thickness or less, some have an allowable thickness of no more then 1/2" or even 1/4".
How unlevel is the floor? Is it unlevel in spots are is it unlevel everywhere?
Do some extensive research to find what you are most comfortable with and best suits your needs. What ever the leveler it must be smooth. Any uneveness will eventually wear through a vinyl floor.