I have changed the final drive ratio on many motorcycles and enjoy doing the math as I go down the road to figure how fast I am really going. It keeps my brain busy. However, I can see why you would want your van to read correctly.
With modern rigs they depend heavily on wheel speed for things like abs and traction control. Others have mentioned shift points in the transmission and possible damage with an incorrect speedo reading. I am not an engineer but this all makes sense to me. Will your abs and TC still work? Sure. Will it work as well as with a 27" tire? I doubt it. Same for the transmission. It needs accurate info to work.
At some point the aftermarket will provide a way to fix this I bet. VC claims a factory reflash was available, I am surprised that they would offer that up if it was not true.