I have had the same problem
After spending much time removing cables, straightening them, greasing plastic parts etc. I decided to set the airflow to windscreen position and leave it in that setting permanently. to do this disconnect the cable from the control knob, then remove panels from near the clutch peddle, to gain access and manually turn the mechanism.
I suspect that the previous owner running my vivaro vehicle into the ground caused all kinds of problems.
dom
December 2016