In a small bowl, mix the sugar, cinnamon, nutmeg, and salt together.
Melt butter in a large skillet over medium heat; add sweet potatoes, and stir to coat.
Sprinkle sugar mixture over the sweet potatoes, and stir.
Cover skillet, and reduce heat to low.
Cook, stirring occasionally, for 1 hour, or until the sauce is dark and the potatoes are candied.
They should be tender, but a little hard around the edges.
Stir in vanilla, and serve.