What vintage cars are we talking about? I have never seen any kind if spring loaded vent on any diesel VW that I have owned. Most of mine have been older stuff though.
MK2 gas and diesel afaik, I just checked my '92 golf diesel and it has it. It's just a little lever inside the neck towards the rear of the car that you push IN and it opens it.
I don't know about anything newer.
I think you need to wax your car. Less wind resistance
I put a gauge on my IP this evening and I had 20 psi at about 1100 rpm. Guessing on the RPM part... I increased the pressure to 43 psi. I did only a short drive and didn't notice a difference in performance. I put my gps in the car and am going to check odometer and speedometer, also upgraded this week, then keep fuel economy and see if it makes a difference.
Rod