**Prepare the Crust:**

1. In a bowl, mix graham cracker crumbs, melted butter, and sugar until combined.

2. Press the mixture into a 9-inch pie dish, ensuring an even layer.

3. Refrigerate for at least 30 minutes while preparing the filling.

**Prepare the Filling:**

1. In a small bowl, dissolve the orange gelatin in boiling water. Stir until fully dissolved, then add cold water and mix. Let cool to room temperature.

2. In a separate bowl, whisk together sweetened condensed milk, orange juice, orange zest, and vanilla extract.

3. In another bowl, whip the heavy cream until soft peaks form.

4. Slowly fold the whipped cream into the orange mixture, then gently fold in the cooled gelatin mixture until fully combined.

5. Pour the filling into the prepared crust and smooth the top.

6. Refrigerate for at least 4 hours, or until set.

**Finish with Topping:**

1. Once set, spread or pipe whipped cream over the pie.

2. Garnish with orange zest and mandarin orange slices if desired.

3. Slice and serve chilled.