In general the Motion Vectors need an alpha channel
About: C4D Object Buffer
http://revisionfx.com/support/faqs/hostfaqs/cinema4d/
Your color/beauty passes should be RGBA as well
If an object is not semi-transparent the MV could share the same alpha
(these alpha channel assignment mean precomp in AE)
You can only generate from CG one direction of motion and you need to figure out with C4D if the motion goes forward or backward (no idea) - so leave the other off (NONE) in twixtor
Pierre