At Basle we had to buy 2 and stick one in the van window. About 4 weeks later I tried to remove it and despite using loads of fairy liquid most of the glue remains. Worse still, the acrylic has crazed. So be warned!!!
Keith - a good tip - stick the vignette to a smooth piece of cling film, then place the cling film (slightly larger than the sticker) onto the inside of the van window. If you are careful it is impossible to detect the clingfilm 'layer' from the outside, and then the vignette is instantly removable after your trip with no mark or damage to the window. This is probably strictly not in accordamnce with Swiss requirements - as they are designed to stick fast to the vehicle and be non-transferable -but I have done this several times now with no problems; after suffering as you did (I never got the whole thing off the window, when I sold the van a few years later it went with it!). Cheers, Richard.