here's how MB can affect a track... all of these blurs can be achieved with different settings in Maya Software motion blur settings (though again, I dunno about Vray's). If you imagine these effects applied to a camera move, can you see how that may cause drifting if you've got the wrong settings for the blur? In my experience, each shot is a bit different, and even if your shutter speed is right, if your shutter settings aren't right, you could come out with one of these below. Which can cause surprisingly bad drift. I'll upload a video showing what I'm talking about later today hopefully. It really surprised me when it happened. Of course, it happened in After Effects, but the end result is similar in all packages.