Any particular reason that you're looking at a Vax 500? Hopefully you are getting a good deal on a used one. The new JTV's are killer!!
Line 6 VDI cable is what you would need. It digitally connects the Vax to your XTL, and also powers the Variax.
You can also just use a regular guitar cable, if you don't mind running off of batteries, or the Vax power supply. You also give up being able to switch models from the XTL though.
I've taken to running mine without the VDI cable, as I prefer the freedom of a wireless rig, and the VDI definitely isn't wireless.