Article 12 of the Toll Road and Toll Regulation requires that invalid vignettes be removed from the windscreen, but there is no penalty for failure to remove them. Some "conscientious" drivers are real collectors of invalid vignettes - the worst are those who have 10 or more invalid vignettes on the driver's side, many of them certainly in the field of vision (today's "winner" is attached).
We have had vignettes in Slovenia for almost 7 years, and neighbouring countries for even longer. Did no one really think 7 years ago what would happen to unremoved vignettes if there was no sanction for non-removal?
Well, now would be a good time to finally sort it out. There is no need to introduce a fine. For example, a regulation could be adopted that a vehicle cannot pass a roadworthiness test if it has one (or more) invalid vignettes (from any country) on its windscreen.
The owner can then remove the vignettes himself or pay the test staff to remove them (some drivers are clearly not up to this simple task, which can be done in a minute with a €1 remover and a damp cloth).
It is true that new vehicles do not need a MOT for the first 4 years (and can therefore collect vignettes for as long as 4 years), but in my experience vehicles that have a 2 or 1 yearly MOT are more likely to be stuck with vignettes.