You do indeed need a yearly vignette to drive on Swiss motorways. It costs CHF 40 and needs to be affixed to the windshield to be valid (in principle, merely having one somewhere in the car isn't enough to avoid a fine). The vignette for a given year is valid from the beginning of December in the year before until the end of January in the year after.
I don't know for sure but I doubt you can buy that vignette from car hire companies as far out as Frankfurt and I would be really surprised if you could rely on a rental having one already (as others have suggested you can always ask or be lucky, but it's not standard).
But you don't need to worry about first entering Switzerland and trying to track down a place that sells it, getting one on the way is really easy. Usually, you can buy the Swiss vignette at the last couple of petrol stations/rest areas in Germany (possibly losing one or two euros on the exchange rate but nothing to worry about if it feels more convenient) and at the border itself, certainly on the motorway (smaller border crossings do not have any customs office/staff nowadays but major crossings still do).