Vignette - think of it as a permit. You need to have it to drive on motorways in Switzerland. Note
that you don't necessarily have to have it to drive on regular streets, but if you don't know which is
which, getting the vignette is the safe way to go.
I suppose it is possible that a rental car in Annecy (France) may have a vignette on it, but if not,
then you buy one as noted.
However, the 40 CHF for the vignette is probably enough to pay for most of the bus fare for two
people RT between Annecy and Chamonix. A day is not enough to see everything, but you could
probably see two or three highlights. It would be imperative to buy tickets for Aiguille du Midi online
and make a timed reservation for entry in advance.