I successfully bid a monthly slot a couple of months ago.

Just looking at it now I see that after selecting "monthly vacation bid"
and click trade it populates the list with your awarded weeks.
If you have consecutive weeks it breaks them in to 7 day blocks which are selectable. If you only offer one selectable 7 day block for another I don't think you could go wrong.

Instructions are in
Flying Together > Pilot > Crew Resources > Vacation > Monthly Vacation