I'm not totally clear on what you are saying; are you saying, that the lock tumbler will not turn enough degrees to unlock the door? The tumbler could be broken; worn out or need to be lubricated. You could take it to a auto locksmith and see what he can do for you or order a new one from your local vw dealer with your vin number (this way; it will fit your current key).
If you want to try to lubricate it; try a product like this, available at your auto parts store. Most seem to be graphite or silicone based lubricants.
Yes. The tumbler doesn't turn clockwise far enough to trigger the lock. I disassembled the whole cylinder but haven't seen anything unusual. except that the tumbler is loose which I assume is intended to be.
Well, think about how many times; it has been used over the years (probably the original part). It could be just worn or wearing out; I had to replace the tumbler for the ignition switch as it was getting harder to turn and sticking. The new one fixed the problem; that maybe the case with yours. Did you try to lubricate it and did that make a difference?
My passenger side door lock/tumbler is fine but honestly, I hardly ever use it (as I use the remote to unlock the door).
A graphite lube would be a better lube; a new tumbler would probably be the way to go. I tried to lube my old ignition tumbler but a new replacement, fixed the problem. In the case of the ignition; a new switch and tumbler, removed any stickiness, binding and was a big improvement. Mine were original and just worn out.