Combine the first 5 ingredients in a large saucepan. Stir constantly while bringing to a boil. Allow it to boil until it reaches the soft ball stage (about 235 degrees). Remove from heat and add the vanilla and the chocolate chips. Stir until melted and smooth. Pour the fudge into two buttered 9x9 inch pans or one 9x13 inch pan. Cool until firm and then cut into pieces. You can use other kinds of chips, such as butterscotch, mint or white chocolate, for a change of taste