Surprised that fewer people don't sort by MP! At one point I sorted by type (i.e., tanks, then halftracks, etc), but the problem is that the counter sheet doesn't show this information, at least for MMP-style black-and-white images. So if I'm not familiar with the vehicle, and I don't know if it is a halftrack or a car or a truck, I have to look in multiple places. Much easier to focus on a single number that is on every counter. I prefer it to the MA because I don't like separating AAMG-equipped Jeeps, etc, from the unarmed ones. (Yes, I know that there are vehicles that have different MP on the front and back, but I sort by the front, and I have yet to come across a scenario that gives you the trailerless version of the vehicle in the OB, so it hasn't bit me yet).
Of course, you run into the problem that 99% of German vehicles have 13 MP, so I sort alphabetically by name after that.