In a medium saucepan, combine sugar, evaporated milk, butter, marshmallows, and salt. Stir over medium heat until marshmallows are melted and mixture comes to a boil; boil for 5 minutes, stirring constantly.
Remove from heat; stir chocolate chips and vanilla into the mixture until completely blended into a smooth, shiny fudge.
Pour fudge into an 8x8 inch wax-paper lined pan. Cool in refrigerator.