Sorry I'm late, but the correct way to successfully scoff a vanilla slice is to turn it upside-down, remove the bottom piece of pastry (which will of course now be on the top), eat it, and then eat the rest of the thing safe in the knowledge that it will not squash out the sides.

This method also works for the things they used to call "French vanilla slices" in Cronulla cake shop in the '60's, but which I've never seen anywhere since. (Never been to France)
French vanilla slices have a layer of cream on top of the layer of custard.

I guess I could say fairly honestly that I've never tasted a better vanilla slice than the French ones they used to sell at the Cronulla cake shop in ythe '60's