Not sure how you get the motion vector pass out of C4D - usually one outputs an EXR sequence (or some other 32 bit floating point capable file format).
But once you have that, it's simple - you just need to merge the EXR's red and green data into the color image's vector channels. Like so:
- Create a loader for both, the normal color image data (e.g. a PNG sequence) and the vector pass data (the EXR sequence)
- Create a ChannelBoolean
- Pipe the color image into the ChannelBoolean's foreground
- Pipe the vector pass into the ChannelBoolean's background
- In the ChannelBoolean's attribute inspector, switch to the "Aux Channels" tab
- Turn on "Enable Extra Channels"
- Look for the "To X Vector" popup, set it to "Red BG"
- Look for the "To Y Vector" popup, set it to "Green BG"
- Pipe the output of the ChannelBoolean into a VectorMotionBlur