I have never Candied Sweet Potatoes in the oven. My mother and mother in law always cooked them in a pan on the stove burner and they were FANTASTIC !! Not ever a bite left. I hope you enjoy cooking them this way. I think long ago when they cooked on wood burning stoves this was an easier way. Also, the oven was reserved to cook a huge pan of corn bread. One of my favorite meals is Purple Hull Peas, Candied Sweet Potatoes and Corn bread...I know, that is down home Southern where the stars shine bright on a cold winter night.
Scrub, wash and peel potatoes, place in large bowl and cut into 1/2 inch slices across. Set aside.
Spray a large, heavy skillet with a non stick cooking spray. Add slices of potatoes and the 3/4 cup of water.
Bring to boil on Medium high, cap with the lid and turn burner down to low. Check occasionally about every 10 to 15 minutes, DO NOT STIR. May gently slide with a wooden spoon always replacing lid. When potatoes are tender, about 20 minutes or so, gently check with a fork. Add, butter, sugar and vanilla. About 1/2 cup of liquid should still remain in pan with potatoes. Recap with lid and simmer until sugar is melted, and butter and vanilla are incorporated with drippings.
The drippings should thicken allowing you to turn potatoes gently with a spatula, don't let them stick. When Juice is thick and cooked real low, remove from burner. Leave lid on potatoes for a few more minutes to allow more steaming. The potato slices should have taken on a candied appearance.