Installing Basement Subfloor over 30-year-old Vinyl
Vinyl is a material that takes millennia to degrade. It is waterproof and is resilient enough to .walk on comfortably. It would be wise to leave the vinyl and build directly on top of it. Typically, when installing a floor in a concrete basement a vapor/moisture barrier is installed to keep the materials above it dry.  Can you clarify why the vinyl would add to the slope (of the floor?)? Are you concerned with the floor being raised enough to interfere with the door bottom? If so, it is relatively simple to trim the door bottom.