If you have a van with factory sliding windows that don't leak it's not a VW!
By the way the windows don't leak it's the drain channels that don't run the water outside. The window seals will always let some wet in sometime during their life but it should be caught in the drain channel and routed outside through the either of the two drain holes. Try using a little funnel and pour water into the front drain hole, I guess that on faulty ones it will run inside not outside. You can't get to the centre hole without dismantling the window but if you could you may find the same problem. Been there, done that and even sent VW a video of what happens and that was at least six years ago but they still send out faulty parts.