While it isn't possible to actually roast the potatoes sous vide, the pre-cooking which would normally be done in boiling water can be done this way, and it will produce much more consistent results. The flavour will be better too as none is lost to the water in the saucepan.
Preheat the water bath to 90°C
Peel and chop the potatoes then season with salt
Place the potatoes in a single layer in a vacuum bag then add two sprigs of thyme and a little fat such as vegetable oil or goose fat
Vacuum seal the bag and place in the water bath to cook for 1 hour
Once cooked, plunge the bag into iced water to cool rapidly and to prevent further cooking
Preheat the oven to 190°C
Heat a roasting tray with a little more fat until very hot and add the potatoes. Roast in the oven for 30–40 minutes or until golden brown and crispy