Combine the sweetened condensed milk and chips in a heavy saucepan over low heat. Heat until chips are melted, stirring until smooth. Add the vanilla and nuts. Pour into a buttered 8x8 inch pan and let sit until cool. This fudge never gets sugary and keeps well. You can make a two-toned fudge by making two batches (one using chocolate chips and one using peanut butter chips).