we spotted a dry rotten line going nowhere.
at least I found out (thanks to this forum) that it is a line from the air vacuum bleed valve. on one side it looks like a small airfilter on the valve and on the other side was this rotten line. is it correct that it goes to this round white part that is broken off? and what happens if I just close it or let it open?

 

The AIR pump gets confusing because the details shown in the factory service manual (6E-908 to 6E-912) were revised as part of a "recall" campaign that covered 1995-1997 models. Details depend on whether the vehicle had the recall work performed.

Shoebox has the recall info, but it's not very clear. The very last two diagrams on page 6E-611 and 612 appear to show the line you are asking about. I can't compare with my 94 because it didn't have any of those components.

http://shbox.com/1/air_pump_recall.jpg

 

I saw the recall info but as you said, it is not very clear.
I assume that on mine it wasnˋt done. It looks like for this you had to tab a new hole.
on mine it looks like this:

 

I found this picture. Looks like they added a T-piece to the line for the EGR-vacuum...
What is this line good for anyway?
For now the line is closed on the switch side and open on the other side (picture 1). Should I just close it all around or route it like in this picture below?

 

The white disc on my car had a little nipple looking thing on it, but there never was any hose connected to it. Are you sure the hose you have rotted is not what is supposed to be going to the opti?

 

I doubt it, because the other end had this nipple in it and some glue at the of the rubber. But interesting that yours is just open.

I will check for sure that there isn't a line missing going to the opti. This would mean that there's a open point and water could go in, right?

 

Look at Rob/Shoebox's AIR recall diagram. In the lower right corner diagram, it shows the “tee” with the EGR vacuum line connected to it and the vacuum line for the AIR system.

http://shbox.com/1/air_pump_recall.jpg

Is your Opti vacuum line connected to a nipple a bit to the rear of the EGR nipple?

http://shbox.com/1/opti_vacuum_conn.jpg

http://shbox.com/1/opti_vacuum_hose.jpg

http://shbox.com/1/opti_vacuum_harness.jpg