I don't think the vacuum retard would have anything to do with idle
emissions.  The initial (idle)  timing could be set anywhere VW wanted it.
On my '85, neither vacuum advance or retard was affected at idle.  (When I
pulled them off and plugged the lines, the timing was the same as before.)
The fact that the retard side pulls its vacuum source from a different spot
should be a good clue.  If I remember correctly,  it gets its vacuum
upstream of the throttle body, which tells me it has something to do with
high speed emissions?.  It would kick in when you get off the gas and
retard the timing until you want to accelerate again?   Anyone else want to
take a stab at the vacuum retard purpose?

                                                        Jay  '85 Westy