VCD and black & white source material

[Hafan]

Hi
I would like to create a vcd with app. 120 min of
b&w video originated from a VHS.
In theory it should be possible to reduce the filesize
if no color information was taken in to account.
Problem is - I can´t find any settings in TMPGEnc to
encode only in b&w.
Anyone know how to do this?