In a 'unibody' (also referred to as 'monocoque') design structure, any skin section is a structurally-stressed component of the entire structure.  A Vanagon is considered unibody, even though it does have frame strengthening members underneath.
Those roof support arches are more for rollover protection than torsional rigidity of the body.   The roof skin, however, is an engineered, diagonal bracing that contributes greatly to torsional rigidity of the entire body, in addition to providing added metal structure to add crash or rollover protection.
  Take a shoe box, remove the lid and grab the ends and attempt to twist it; it will easily deform.
  Now, put the lid on and repeat; you'll have a hard time deforming the box from it's basic shape.
  And, now, for the final proof of the body weakness created by a large, unreinforced sunroof opening; cut a good -sized 'sunroof' into your shoebox lid and repeat your twist test.  What happens?  My bet is that a great amount of overall rigidity is lost due the 'missing' area.  You've cut the center out of your 'diagonal bracing!  
  VW made up for it by adding 'doublers' inside the factory sunroof, not just a couple of cross ribs.  The probably also added outboard, lower frame cover plates (I know they did on the earlier buses).  Side impact crash protection, must also be taken into consideration in this modification.  (Smack the side of the shoebox to demonstrate this aspect.)

Mike B.

  As long as you don't cut the 2 main roof arches, one at the front edge
  of the slider door and one at the rear edge, the roof area between is
  just roof skin and not of much importance structurally, IMHO.