Is it OK to install vinyl flooring directly on top of a cement slab.

Any little rough spot will soon show through. You can eliminate that with floor leveler sold where flooring is sold. You can also fix low spots with it.

Also make sure to clean the surface... then clean it again... then one more time. After it is claen and dry (since mopping was in order) use a primer agents. They sell it near the flooring in your local home store.