Double Stud Wall interior vapor barrier with Gussets – GreenBuildingAdvisor

I am building a double studded house (9′-9′.6″ walls) in central maine zone 5.   I am wondering how you effectively install a vapor barrier on the exterior of the interior wall when there are gussets connecting the interior and exterior walls.  The designer has the walls as follows from inside to outside:

1/2″ GWB…….DOUBLE 2X4 EXTERIOR WALL (TOTAL WALL = R44), ~ INNER 2X4 WITH MINERAL WOOL INSULATION (R15), ~ SIGA SWISS MAJREX 200 V.B.,  ~ 4.5″ SPACE/INSUALTOIN GAP BETWEEN STUD WALLS. PLYWOOD GUSSETS AS REQUIRED.,  ~ 2X4 KD STUDS AT 16″ O.C. WITH DPC OR WOOD FIBER INSULATION IN 2X6 AND GAP CAVITY (R29)…….

EXTERIOR 1/2″ PLYWOOD SHEATHING – SEAMS TAPED TO CREATE A.B. NO OSB WALL SHEATHING,  SIGA SWISS MAJVEST 200 WRB – S “WATAIRVENT” HORIZ. STRAPPING – RAIN SCREEN WOOD COMPOSITE BOARD AND BATTEN SIDING – LP SMARTSIDE “MIDNIGHT SHADOW.”

I plan on using Dense Packed Cellulose throughout instead of combo of wool and DPC.  Unless people are opposed…

Questions:  1.  How do you have a VB on the exterior side of the interior wall with gussets?  I assume the interior wall needs to have the VB installed and wrapped around plate/header and taped prior to standing and nailing into place.

2.  With DPC, I would assume with a 11.5″ cavity that DPC needs netting between every other bay to achieve the 3.5-4 pounds per cubic foot?  Not sure how this is achieved with a VB on the exterior or the interior wall.  And is the Marjex200 sufficient to withstand the installation of the DPC to achieve the pounds per cubic foot?

3. Is the VB needed on the exterior side of the interior wall?   Or better to place between the Drywall and studs or not needed at all?

4.  Or if the VB is required on the exterior side on interior wall, should blocking be used instead of gussets to prevent twisting, but still an issue with netting bays.  My current house, the studs twisted, so blocking or gussets I believe should be used on this house.