Stephen Wylie blogged about his Turbo Vectrex controller build:
The main impetus for this was to have a homebrew controller that actually featured an analog joystick, since there were few if any guides elaborating how to fashion one from an existing controller. I acquired a couple Parallax 2-axis joysticks with breadboard mounting capability to do the trick.
The Vectrex comes with a game in its ROM — Asteroids — thus you can play without needing a cartridge. However, with the traditional controller, this requires lots of button-mashing since it has no auto-fire feature. Using a 555 timer, potentiometer, and clever values within an RC circuit, I have given it the ability to auto-fire.
See the full post on his blog here, GOSHtastic.