While not absolutely required, most duck leg recipes call for using the confit technique. This involves an overnight brine and then cooking them in duck fat. Sous vide is great for duck leg confit in that it requires much less fat than required for traditional confit.
Duck Leg Confit (Dry Brine)Food and Wine
Duck Leg Confit (Wet Brine)The Foodie Cooks
Wet and dry brines both work fine for this, but make sure you rinse the duck legs thoroughly.
The legs will cook in their own fat within the bag, but adding duck fat still adds a lot to the flavor. If duck fat is not available, try using butter or olive oil.
Sear each side for about 20 seconds over high heat to get the skin nice and crispy.