There's a long thread on Samba about that piece ("plenum") and I'm the first one to suggest removing it for better airflow.  That thing is hollow inside except for a couple of molded steps that permit screws to hold it to the ceiling.  One guy added foam blocks to the inside to channel the air directly to the vents you mention for better airflow, but removing the thing entirely gives a blast of air to the front seats.
Search on "plenumectomy" and enjoy the reading.  If you know who TK is you'll appreciate my warning about the entertainment value of most of the pages.
Stephen 

    On Monday, January 9, 2017 9:14 PM, Neil2 <vidublu@GMAIL.COM> wrote:
 

 OK folks, time to earn your keep!

My '90 Westy has the rear A/C shroud with 6 forward vents and two down.
Can anyone tell me how the air is ducted from the rear of the plastic
shroud to the vents in front?  Materials, shapes, sizes?  Directions,
angles?

I spied an aftermarket 'improvement' which eliminates the plastic and
places a single, flat vent on the vertical face, that supposedly improves
air flow to the front.

Please/thanks!

--
Neil2
'82 Diesel Westfalia (Ducky)
'86 Vanagon (SaVannah)
'08 170" 2500 Mega Roof Sprinter (Moby)
'90 Westfalia Subagon (SaL)
Fulltiming since August 2008
Nunquam Pendite Divendium