It is VERY important to start the 5 minute countdown when the mixture is at a full rolling boil. It should look like this.
After 5 minutes turn off the heat, pour in the marshmallows, chocolate chips, and then the vanilla. Stir until combined and melted. Add the pecans. Stir until combined. Pour the fudge into the prepared pan. Let cool for 15 minutes or so then cover and place in the refrigerator for 2 hours or until firm. Lift fudge from the pan and cut into 1 inch pieces as needed.