Wash and dry apples. Remove stems. Insert a wooden skewer into the stem end of each apple. Melt butter in a heavy 3-quart saucepan. Stir in sugar, corn syrup, and salt. Bring to boiling over medium heat, stirring constantly. Stir in condensed milk. Cook, stirring constantly, to 245 (firm ball stage), for 12 to 15 minutes. Mixture should boil gently over entire surface. Remove from heat and add vanilla. Dip each apple into caramel mixture, turning to coat completely. Set on a buttered baking sheet and chill.