From the FAA Pilot-Controller Glossary:
VISUAL DESCENT POINT A defined point on the final approach course of a nonprecision straight-in approach procedure from which normal descent from the MDA to the runway touchdown point may be commenced, provided the approach threshold of that runway, or approach lights, or other markings identifiable with the approach end of that runway are clearly visible to the pilot.
I think the VDP concept was more relevant to the old 'dive and drive' NPA technique. After you levelled at the MDA, the VDP was a useful guide to where you should restart your descent. If you were still level after the VDP, you were probably too high to make a normal landing.
With the general acceptance of CDFA techniques, the VDP is perhaps not so useful anymore, except for special cases where the CDFA may not be appropriate.