I'd guess in 2026 all vignettes can be bought online, for several different periods from one day to whole year, so you can and should buy them once your itinerary is set.
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and a government website that has a downloadable word document that explains vignettes for foreigners but is 14 years out of date and tells you to buy a ticket at an office that has been shut since 2008.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.