Surely it's because I'm a bit daft, but what exactly is the question? Is it how to interface a potentiometer shaft to a vex shaft? Or how to connect a pot to the Vex controller? How to assemble a ready-made pot for Vex folks to use? Or something completely different?
I'm not trying to be sarcastic or anything - I'm just puzzled as to what problem you're trying to solve
Don