A good question. My guess is that a Vix-Ant is a cross between a civilian Vixen and the Ant style wartime cab. The Ant, whilst using Vixen components, also had, for example, larger section tyres.
As far as I'm aware the Ant only came in short wheelbase form and supplied principally to Government departments, whereas the Vix-Ant was supplied to the wartime civilian market.
Lots of interesting Guy information here:
http://www.localhistory.scit.wlv.ac.uk/Museum/Transport/commercial/Guy/guyhistory4.htm
http://www.localhistory.scit.wlv.ac.uk/Museum/Transport/commercial/Guy/guyhistory5.htm