OSB or tongue and groove boards for flooring in an expandable attic?

Hi! Basically, I am facing the same question: - The ceiling and roof slopes are insulated and have a vapor barrier - The attic is a "cold roof," with an underlay beneath the tiles and 100mm core drilling on the gable sides for air exchange My thought now was to also insulate between the rafters in the roof to somewhat balance the extreme temperatures in summer and winter. But now I have the same questions: - Do I have to install a vapor barrier in the attic? - If yes, then it should be as "tight" as possible, otherwise, I might as well not do it - But it is rather difficult to do this in the attic; if anything, it would rather serve to keep the insulation material in place so that nothing falls out (disgusting...) - What happens if I don’t install it? ... The insulation will probably get damp because of condensation ... But it should dry out again... Isn’t there anyone who has already dealt with these thoughts/this project and can report back? Thanks to you all and best regards!