Probably would have been an as movement to, I think all the slim vietnam cases take that movement, over the last couple of years they have become hard to come by.
This is the kind of thing,the old skin diver type of plexi. Really the dsn rehaut with the gasket is designed for a sapphire crystal but I used one of these in one of my dsn builds and it was pretty water tight , my Vietnam blancpain builds have the diver tite crystal and even without having a screw down crown they are water tight
Just measure the crystal and then get a tension ring crystal from cousins, get the same size as the dsn plexi to fit inside the gasket, the tension ring makes for a tighter fit, in an ideal world you would fit the tension ring crystal without the gasket but if you do that the extra size will mean that when you turn the bezel you will turn the crystal
https://www.cousinsuk.com/product/divers-chrome-ringed-glass-atct