Heat the marshmallows and 1 1/2 cups of the heavy cream in a pot over medium heat, stirring gently, until the marshmallows are completely melted, about 4 minutes. Set aside and allow the mixture to cool to the touch, about 20 minutes.
Meanwhile, make the crust. Pulse the graham crackers in a food processor or crush using a rolling pin in a resealable plastic bag with the air taken out until they are evenly sized crumbs. Transfer the graham cracker crumbs to a bowl, add the sugar and mix until evenly combined. Next, pour in the melted butter and fold the mixture until the butter is distributed. It should be the texture of wet sand.
Transfer the crumb mixture to a 9-inch pie pan, reserving a few pinches for garnishing the finished pie later. Press the crumbs into the bottom and sides of the pan to form a crust. Some will come up and over the top of the pan.
When the marshmallow mixture has cooled, add the vanilla to the remaining heavy cream. Whip the cream by hand with a whisk or in a stand mixer with a whisk attachment to soft peaks. Gently fold the whipped cream into the cooled marshmallow mixture.
Transfer the filling to the prepared crust and smooth out with a spatula. Sprinkle with the reserved graham crumb mixture and dot with the maraschino cherries, putting one directly in the center. Chill until set, about 3 hours.