Vapor barrier connection

Hello I need help with the connection of the vapor barrier. I have already renovated the side rooms in my old Appenzeller wooden house. Now the attic is next. I have sketched it on the two attached drawings (section through rafters and section through the main purlin). However, I do not know where to connect the vapor barrier. The main purlin is covered by about a 30mm thick rough plank and I do not want to remove it because it is in good condition, sits cleanly, and also forms the ceiling of the lower rooms (bathroom and bedroom). Can I apply the vapor barrier airtight to the rough plank, or are moisture problems inevitable? I also have this problem at the gable walls at the bottom. Here I cannot remove the rough plank because the beginning and end of the rough plank rest on the gable sills. However, I think it is best to apply the vapor barrier airtight as close as possible just above the rough plank before installing the insulation and then pull it upward after insulating. I am very grateful for any answer! Regards, Nick