An acronym can mean different things to different people however in the context of VDP I would wager it is a planned descent point. A VDP is a charted point where a PDP is basically the same idea however just made up by the pilot.
Ill give you my theory so take it whats its worth and modify it to fit your needs.
Ideally you want to be established on a 3 degree glide path for landing - correct? Also FARS say at DA you need to have reqd visual ref, visibility, and continuously in pos to make a landing in tdz.
When all else fails , especially in darkness, and poor visibility which can be very disorienting this is critical!
If tou devide the HAT by 300 you get pretty close- Por Ejemplo
lets say tdze is 100 feet and MDA is 750 feet msl- your height above hat is 650 that you need to lose from mda to touchdown.
I suggest planning on crossing threshold at 50 feet so really only need to lose 600 feet Divide this by 300 and you come up with 2.
That is 2 Miles from the threshold where one should leave the mda and if you maintain your normal descent rate(approx 5 x GS) and you should be well established.
If timing were used to figure this vs DME then you use a similar formula. For exaple if in a plane that has approach of 120 kts divide by 10. (600 divided by 10= 60 seconds) 60 sec before map one should be at pdp.
In large or wide body ac instaed of dividing by 300 I would use 325( a 3 bar vasi-upper path is 3 1/4 ) for wide bodies
Finally in faster planes I would also incorpoate a push over point
SAy we have a vdp at 2 miles and we wait till exactly 2 miles to start descent we will be at about 1.8 before we begin our descent so now we are high-- I generally have a push over point about .2 miles previous to my pdp-- ( this is probably only necessary at speeds above 120)
