In a heavy duty saucepan over low to medium heat, melt chocolate chips and sweetened condensed milk. Remove from heat and quickly stir in salt and vanilla, Spread evenly into the waxed paper lined pan. Chill in the refrigerator until firm (about 2-3 hours). remove from refrigerator and cut into small squares. Place fudge in air tight container and store in refrigerator or at room temperature if serving in a day or two.