Butter 8 inch square pan; set aside. Combine sugar, eggnog, corn syrup and butter in medium saucepan. Cook over medium heat, stirring constantly until sugar dissolves and mixture comes to a boil.
Add candy thermometer (bulb of thermometer should not rest on the bottom of the pan). Continue to cook until mixture reaches the soft ball stage (238 degrees). Pour into large heat proof mixer bowl. Cool to lukewarm (about 110 degrees).
Add vanilla and beat with electric mixer until thick. Spread into a 8 inch prepared pan. Score fudge into squares with knife. Refrigerate until firm. Cut into squares. Store in refrigerator.