Using Hardwood as a Subfloor
Never say never, a low perm flooring like vinyl may perform successfully when installed over hardwood. But unless the person installing the floor has a better than average understanding on how such an encapsulation will affect the finished hardwood flooring that is now relegated to subfloor duty they’d be wise not to take the risk.
Subfloor Ramps
The floor does not need to be level in most situations, but should be flat.