Have a look at the speeds Nuthinondaclock. Not untill the A/C levelled off at 1500 feet and 12 miles did the speed even come within a bulls roar of vyse! If vyse was adopted from the biginning, the ROD would have to have been less, and probably significantly less.
I realise from ATC data that they were groundspeeds, but lets say vyse was adopted from the beginning and the A/C was experiencing a 50-60 knot tailwind at altitude. If the IAS was at or around vyse and the ROD's were around 700-1000 FPM then I would suggest in a PA31 you are essentially a glider, and any rational person would have told ATC such.
Whilst I agree that more info is needed, I cannot for the life of me conclude anything other than the pilot did in fact throw away altitude.
Hopefully the investigation may determine why?