Yeah, if it's hitting -20, I'd say even the unprepared diesel runners might find things a struggle.
WVO is simple enough down to around -2 or -3, with moderate preparation. Any lower than that, and you've got to get it right, or the vehicle just won't go.
I'd say, if your consistently dipping below -6ish, you'd be looking at a heated fuel tank, unless you want to be adding large percentages of diesel. It would probably be more like adding small percentages of WVO, actually.
The key to WVO in cold weather is perfect starting system, Batteries spot on, wires and cables all clean, and heater plugs, if you have them, all working. Putting larger (8mm or 10mm) fuel lines helps a lot, on startup and running. It gets the thicker WVO through easier.
The other thing to think about, WVO is an utter swine to filter, even when the temp goes below 10C. What was once nice liquid wvo suddenly starts turning into semi solid gunk.