I'm building a conventional warm roof which is grp on 18mm osb3 on 200mm pir on vapour control layer on 18mm osb3 on firrings on I-joists on plasterboard.  I can't see how you can get the vcl down from the lower osb3 deck to the walls to provide continuity of the vcl with the vcl in the walls.  Seems impossible.  How have others achieved this?  On a previous project I put the vcl under the lower osb3 deck and then with a bit of effort I trimmed the vcl in the walls up between every joist and tap