The original Stunt Cycle game had a twist grip controller. Here’s a picture of the home version of Stunt Cycle.
Here’s a picture of the Arcade machine
To hook up something like this to a Vectrex you need a potentiometer as it will act in place of the joystick. I hunted around and found this potential candidate. Which has a return spring (it’s quite strong which is good)
Hopefully I’ll be able to make a controller similar to the home game one. I’m working on the analogue and digital acceleration mechanics next so will hopefully have something to test with.