I believe enough breedings have been done (by the Sutherlands) between an alleged het VPI male to het SK females without producing any axanthics to reasonably prove that the SK and VPI lines are incompatible (as long as their VPI het male was legit, anyone know who ended up with it and if it later proved het?).
I seem to remember hearing that someone tried crossing VPI and Jolliff and also failed to produce axanthic but I'm fuzzy on the details (het X het?, how many babies?) and I'm even less sure if crosses have been made between SK and Jolliff lines. Hopefully as axanthics become more common someone will breed actual axanthics from the different lines together rather than working with hets. I realize this would be hard to do on the real chance that they would only produce normal looking double hets but heck, maybe in 20 years or so I can try it ...