You are right, a VDP is depicted on the chart. A VDP is a defined point on the final approach course from which normal descent from MDA to runway touchdown point may be commenced (provided you have adequate visual referece, etc.)
But not all non-precision approaches have VDP's. Thus the question- where can you begin your descent from the MDA on those approaches? You need to calculate a PDP, Planned Descent Point. I recommend getting a copy of one of Ronald McElroys books, "Airline Pilot Technical Interviews" or "Mental Math for Pilots."
There are 2 main ways to calculate a PDP. (I am sure there are others...)
1. The DME Method. Divide the HAT by 300. This will give you the distance in NM to start your descent from MDA to the runway with a 3 degree glidepath (300 feet per nautical mile.) For example, the HAT on the chart says the MDA is 401 feet height above threshold. 401 divided by 300 is about 1.3. (Easy mental math- 300 ft is 1.0 miles, 400 is 1.3, 500 is 1.7, etc., 600 is 2.0....) Your VDP is 1.3 NM from the threshold of the runway.
This would be very easy to see if you had FMS on your airplane, telling you your distance to the runway. But if you don't, you need to do some additional math. Depending on where you are getting your DME info, and where that Navaid is located- you need to find the DME reading at the runway threshold. It may be depicted on the chart, or you may have to look in the timing box on the approach plate to find the distance. Then, add or subtract this distance to your earlier calculated distance (in this case, 1.3), and that is the DME reading for your PDP. Clear as mud? Hard to do without examples. (Get those books I mentioned!) Here's another way.
2. Timing Method. Divide the HAT by 10. This gives you the time in seconds required to descend from MDA to the runway at 600 fpm. So, if the HAT is 401, divide by 10, 40 seconds. Look at the chart and find your timing for your MAP. Then, subtract your calculated number (40 seconds here) and that is your PDP. Please be sure you use the appropriate gound speed to make your calculation accurate.
A lot of this good stuff is n the AIM. A good read before an interview. Good luck!