Arkadiy Demchenko has a four part maya tutorial series on rendering motion vectors from Maya using Mental Ray with multiple techniques. Being able to render motion vectors from a 3D application is always a crucial part production, allowing you to calculate motion blur without actually rendering it, and using the resulting map to apply the blur in post – which always ends up being a faster render.
Also, Ashraf Aiad has a video tutorial on getting motion vectors in Maya by using Mental Ray’s Production Library Shaders. For me this always worked out to be the quickest, simplest solution.