Only the first map defined in a module can be docked to the control window. Check if there is another map defined before the one you want with a Function key to open it. You will need to move the map you want docked up above any other maps.
It is a requirement for every VASSAL module that if you want a map window to be the one to dock onto the Chat window when the ‘Combined’ preference is set, then that map window must be the first map window defined in the module.
You are confusing Boards and Maps. VASL has one Map Window defined containing many selectable Boards. I can’t find the module on the Vassal site, so I cannot help you. Why don’t you email the module designers directly?