They are not specific per canton - one vignette will cover you throughout Switzerland.

I believe they are based on the calendar year, ie 1.1.2010 to 31.12.2010, but if you have a 2009 vignette it will still cover you until May 2010. Makes sense as a lot of people won't get their bikes out until it warms up again. However, not 100% on this, but I'm sure ChrisW will be along shortly with the definitive answer