In a large saucepan combine the first four ingredients. Cook over medium heat until all sugar is dissolved. Gradually add milk and mix well. Continue cooking until candy thermometer reads 248F (firm-ball stage). Remove from the heat; stir in vanilla until blended. Fold in the pecans. Drop by tablespoonfuls onto a waxed paper-lined cookie sheet. Chill until firm. Melt chocolate chips and shortening in a microwave-safe bowl or double boiler. Drizzle over each cluster. Cool.