Beat together the sugar, 2 tbsp flour, egg, and vanilla in a large bowl.
Stir in the cut rhubarb.
Pour into the unbaked pie crust.
In a small bowl, mix the flour and brown sugar together.
Add the butter and use a fork to "cut" the mixture into crumbs.
Top the pie with the crumble.
Place the pie on a baking sheet (this is to help with possible spill over) and bake in the preheated oven for 1 hour or until the filling is bubbling and the top is golden brown.
Let the pie cool, serve with ice cream or whipped cream.
If the edges are browning too quickly use aluminum foil to cover the edges. Alternately, use a pie crust protector (like [easyazon_link identifier="B005FYC9XM" locale="US"]THIS ONE[/easyazon_link]). They work great!