In a lidded frying pan, saute the leeks in the butter until wilted. Add vinegar, salt and pepper; cover and cook on low heat until leeks are tender (about 30 minutes). Remove lid and increase heat to moderately high; add cream and cook, stirring, until cream is absorbed (about 1 minute). Remove pan from heat and cool slightly. Spread the goat cheese evenly over the bottom of the tart shell. Spoon the leek mixture evenly over the cheese. Bake in a 350 degree F. oven until heated through and browned on top (about 15 minutes).