No vignette, because agricultural vehicles fall under the LSVA rules and use these instead. The LSVA / PSVA is required for ALL roads in Switzerland, not only motorways and expressways.
You can calculate and order the required Heavy Vehicle Charge (LSVA/PSVA) here.
Related articles:
What is the Swiss PSVA / LSVA?
Where can I buy the Swiss PSVA for foreign vehicles?
How much does the Swiss PSVA cost?