Forgive my ignorance, but what happens if I don't pay a couple of the installments, for example if I make a major purchase and have a temporary squeeze on funds at the end of this year? Do I just end up paying whatever is missing as part of the shortfall at return time, i.e. in a few months, or is there some kind of additional penalty?
I am resident in Vaud on a C permit and pay my tax monthly using the precalculated orange payment slips. I presume my experience is the same as most people - my income fluctuates, and so these payments are 'guesstimated' by the tax authorities at the end of the previous year. Then the authorities send me a demand for anything from a few hundred to a couple of thousand francs of shortfall in the wash-up when the return is calculated (via a fiduciaire, usually about April/May the following year).
Try contacting them. In my dealings with them they're pretty relaxed as long as they get the money for the year in at some point. A phone call to their offices to explain the situation should help.
They may charge you late payment interest, as the monthly payments do have a due date. However with Swiss interest rates being what they are, it's unlikely to be a big deal.
As the previous poster suggested, it's probably better to discuss with them. They're generally friendlier if they know in advance.