There is nothing stopping you fitting 20” alloys on your T6.
Have a search in the wheels section of the Forum, there are more discussions in their than you can shake a stick at.
Personally I have seen a T6 sitting on 21” rims. As to what VW have told you is based upon them only selling 18” rims as the maximum size as they determine that these are the most appropriate in terms of not causing too great a stress on the components of the drive train. To VW it’s all about the £figures in minimising any potential warranty claims. If they can say we are not paying for your driveshaft fault or gearbox fault because you have fitted oversize wheels, then they will. However, they have to prove that the failure can be directly attributable to your wheels. For example if you take your van in because of a leaky window, this has naff all to do with wheels/ drive train stresses, so they can’t refuse to fix that issue under warranty.
Equally there is nothing to stop you running 20’s, and keeping the original rims in the garage to stick back on it to take it in for warranty work on drive train components.
The irony being is that I have seen 20 inch rims on a van sat on a VWCV forecourt! How the hell can they refuse a warranty on that van?