We will hire a minivan from the airport (Geneva) to our home in Lausanne. Do I need to tip the driver? If I do or should, what would be a reasonable amount or percentage of the fare?

Thank you very much.

No.

Only if he's American.

Certainly not.
Don't worry, as soon as you have seen the bill, the question will naturally disappear anyway .

PS: Eventually, you can give them a voucher for UBER, they like that very much these days!