Well, I'm no expert but can give you my experience with such a setup. I found the rear Suntour VX to function perfectly fine on my Fuji of the same era and wanted to keep it because it was pantographed Fuji. I converted my Fuji to a 650b setup and kept all the stock components except for the brakes. Had to switch to long reach ones. I had no problems with the RD working just fine with the stock DT shifters. It was covering a modern 8 speed cassette with a new chain so I'm sure that helped. If it was me I'd give the old RD a good clean and service and then put it back on. Much nicer look on the bike than the Shimano and Suntour is quality gear.
The pic was shot before the new chain and cables were done.