You would need to assign monochrome.ctb as the plot style table for the layout which is being printed. Before calling vla-plottofile;
1) Obtain a reference to the active layout...
2) Assign monochrome.ctb to it...
(setq objLayout (vla-get-activelayout objDoc))
...Then proceed with plotting.
(vl-catch-all-apply 'vla-put-stylesheet (list objLayout "monochrome.ctb"))
Note that the above assumes the drawing is set for colour-based plotting, rather than style-based (otherwise, you would be using monochrome.stb). Also, you would probably want to include the above in an undo group, and have a call to the Undo command at the end of your function, so that the original plot style would be restored.