Make the crust first (or you can it while the lemon gelatin is setting). Combine crumbs and sugar. Add melted butter and mix evenly. Set aside about ½ to ¾ cup to be used as topping, then press the rest of the crust into the pan.
Dissolve gelatin in ½ cup boiling water. Stir in ½ cup ice cold water, stirring until ice melts. Let cool until slightly thickened. No need to chill, and it doesn't have to be super thick, just slightly thickened.
Beat the softened cream, sugar and vanilla together until well blended, then stir in the fresh lemon juice. Add the thickened lemon gelatin and stir until incorporated. If for some reason the mixture seems too wet to fold in the whipped topping, you can chill it for 30 minutes, but if you let the gelatin cool long enough it should be okay to fold it right in. Since it sets in a casserole dish it doesn't really matter.
Pour over crust and scatter with reserved crumbs. Refrigerate for 6 hours or overnight.
When it's time to cut it you might find the crust sticks to the bottom of the cold pan. An easy trick for getting it to slide off is to set the cold dish in a rimmed baking sheet filled with hot water. The heat from the water softens the cold butter just enough to loosen that cookie crust from the pan.