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.