VfW API for decoder - example?

Can anyone point me at an example of how to use the VfW API for xvid
decoding?

I'm having trouble getting it to work, but if I make a one-line change
to make my
code use DivX instead of AVI, it works fine.

In each case I am generating the input data using the xvid encoder
through the VfW API. 
So I'm fairly sure I'm doing things roughly right, but there might be
some subtlety I'm missing.
The call to the xvid decoder shows no error, but does not produce any
output. Replacing it
with a DivX decoder and it works fine.

Any ideas?
thanks
i.