Us Americans (pun intended) have no idea what “vignette” means in this context. And our dictionaries don’t, either.
They’re usually inexpensive. Most countries have clear and repeated signs telling you you need one, etc.
Slovenia - last I was there - don’t have a single sign on the way into the country. They don’t tell you at the toll post on the border.
What they do have is cops with binoculars sitting in the median flagging down every foreign plated car, and then issuing a fixed €200 fine (they’re ready with a card machine), and telling you you have to buy a vignette at the next gas station, which is where they’re sold.
Anyway. Wildly irregular across Europe. Useful site!
And yeah, they don't tell you. If you're driving europe cross country, best get your armchair lawyer book out and plan on strategically buying them as soon as you enter the country.
If you know that you need them - hence this site is useful. Unfortunately you’ll often look it up as a tourist, you’ll be hit by ten phishing sites and a government website that has a downloadable word document that explains vignettes for foreigners but is 14 years out of date and tells you to buy a ticket at an office that has been shut since 2008.
Plus… sometimes the solution is non-obvious. I live in Portugal. I want to drive my EV into Spanish cities. I need a special vignette for that. Spain don’t issue it to foreign vehicles. Portugal don’t have a scheme. But France do! And it’s reciprocal with Spain! So the solution for Portuguese EV drivers is to get a vignette from France that works in Spain.
Madness, all the way down.