Hi There, you need a Haynes Manual.
That bit is the idle control stabilization valve. It bleeds air past the throttle body to raise the tickover when the engine is cold and when the alternator or power steering has load on it. It should buzz when you have the ignition switched on. It simulates just cracking open the throttle a bit. It has a relay block behind the rear right tail lamp.
Your poor running could be down to a few things.
The track on your air flow meter might be dirty at a particular spot, giving lumpy running. It's a bit technical to do if you are very inexperienced, but should only take 30 mins if you are confident.
The injectors might have a poor spray pattern. This would cause lumpy running, but would be for most of the time.
The fuel pressure regulator might be on it's way out.
The fuel filter could be full and need changing, if the van has not been run for a time regularly then it will have all sorts of muck in the bottom.
My answer for everything now a days is Rolling road tune it, then any problem areas will rear there heads and will be easier to identify.
The last idea is wiring, wiring and wiring. The brain (to be kind to it) of the 2.1 ecu is a solid state thingy which takes readings from various inputs, and then gives certain outputs for fuelling. All these are pre determined by the technicians at VW in the early 1980's, it works great if everything is within vw tolerances, but cannot cope with things being outside the norm, no tell-tale light or plug in diagnostics. What I am getting at is that if you have one corroded terminal from any of the engine sensors then the whole thing will run rough. It is worth getting out the wd-40 and going over all your wiring, cleaning it as you go.
If you search on the above topics you will find some useful info on diy remedies.
I always use Shell V Power if I can get it, it keeps the engine clean, and this is a very high compression engine which needs it to get it great fuel economy
Giving the engine bay a good clean with WD-40, and keeping it well dosed helps prevent rusty terminals and intermittent probs.
Good luck
Simon.
here is another post on the subject....... just do a search at the top of the page for more.