Until you get well into the subzero temperatures, warming up the vehicle is mainly to allow for the defroster to be effective. The oil in the engine is doing its job within 10-20 seconds even in normal winter temps.
-20 and below if the vehicle was left outside you need several minutes even with an engine heater to allow a little thawing in the transmission. Drive and shift easy until all the moving parts have a chance to thaw out.
Metal gets very brittle in extreme winter conditions. I saw a hole blown in the bottom of a V-12 diesel you could put a basketball through when we tried to cold start our tanks in Germany one winter.
