If I remember from my time in Europe the key difference between the vignette and something like an EZ-Tag is the vignette isn't necessarily tied to the registration of your vehicle like the ones in the States are. You can usually walk into the gas stations in europe and just buy them for periods of time like you say.

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.