Coat a 9x13-inch pan with grease.
In a large heavy saucepan over medium heat, mix sugar, margarine, and evaporated milk, stirring until sugar dissolves. Bring to a rolling boil and cook for 5 minutes, stirring constantly, until slightly thickened.
Take off heat and add chocolate chips; stir until melted and fully incorporated.
Fold in marshmallow cream until blended. Stir in walnuts and vanilla.
Pour fudge into the greased pan and spread uniformly using an offset spatula, smoothing the top.
Allow to cool for at least 1 hour before slicing into squares.