I'm guessing you don't have the proper alpha in the vectors pass, and that areas outside the object have zero motion (yellow in the output vectors), with full alpha, rather than 0 alpha in areas outside the object in the vector pass. Are you turning off any background static objects or environment when rendering the motion pass?
In any event, you probably want to read this:
http://www.revisionfx.com/support/faqs/motion_vector_FAQs/motion_vector_alp...
You can always move this the RE:Vision Effects forum if you have further questions, etc.
Pete Litwinowicz
http://www.revisionfx.com