Stir together first 7 ingredients in a 6-quart Dutch oven. Place over low heat, and cook, stirring often, 40 minutes. Using a pastry brush dipped in hot water, wash down any sugar crystals on sides of pan. Cook 15 more minutes, without stirring, until a candy thermometer registers 234° (soft ball stage). Remove from heat; add vanilla. (Do not stir.) Cool mixture to 110° (about 13 minutes). Stir glaze until mixture is thickened, but still flows easily.