In a medium saucepan, dissolve sugar in evaporated milk. Add butter (cut in pieces) and corn syrup, and, stirring constantly, cook over low heat until butter is melted. Continue cooking, stirring frequently, to soft ball stage. Remove from heat and mix in peanut butter. Cool to about 120 degrees F and mix in vanilla. Spread in suitable size pan (the larger the pan, the thinner each piece of candy will be; I use a 8x8 pan). Fudge will still be very soft at this stage; it takes a few hours to set up.