Apparently it can be done. Just read this post by Rob Mitchell on
Atariage:
http://www.atariage.com/forums/topic/66271-vectrex-question/
"...
I have modified the deluxe sticks (both Tandy and IBM) to work with
the Vectrex. They require 10k resistors to be added to the two outside
tabs of the two potentiometers. Four tabs .. four resistors. The other
ends of the resistors are connected to either +5v or -5v. The middle
tabs are connected to the horizontal or vertical lines. I made my
deluxe stick such that the two buttons are at the lower left when
holding the stick. This works for me when I am playing Moon Lander on
the Vectrex!
For the other buttons I use a splitter that allows me to plug two
controllers into one port. The other controller is just four buttons.
So with both I can have full Vectrex control functions.
Cool feature of the Deluxe Sticks is that if you hold the stick in a
corner and flip the two tabs on the bottom to FREE .. you will disable
the center springs.
And for those who wish to service their original Vectrex
controllers .. there is a how to open guide on
www.playvectrex.com.
Also note that a broken spring can be replaced with an Atari 2600
cartridge door spring!!
..."
Kev