If you rent the cars from the airport, then when you enter switzerland at ferney voltaire, you wont have to pay the vignette. Simply follow the signs (green signs ) to france on the motorway and it will take you through Bardonnex customs and onto the A41 to Annecy. However, on the way back if you enter Swiss territory through Bardonnex you will have to pay the vignette, so therefore you need to follow the motorway (a40) to vallard, just some 10km further on, enter geneva through vallard, cross town staying close to the lake and leave geneva towards the airport. then cross into ferney voltaire but just as you cross the customs you will see the sign to the airport french side.