I think the famous area for lavender fields is in Provence - generally around Aix-en-Provence I believe. Those fields are always in the countryside (obviously). So you would probably need to rent a car unless there is some sort of organized tour. I drove around some lavender fields a few years ago, and it seemed everyone had a car. And t o really get good pictures you need to be there near sunrise or sunset in my opinion
I would suggest you take a TGV from Paris to Avignon (2.5 hours I think). Stay there, and rent a car to visit the area (Nimes, Port du Gard, Arles, Les Baux, Roussilon, Aix, La Coste, etc.) It's a famous and wonderful area. You could even go to Nice, Cannes, Toulon, Menton, Antibes, and Monaco not much further.
I would say that Paris and Provence is a great vacation for 10 days.
But if you really want to hit Lauterbrunen too - which is also great - I think the fastest way to take a train from Avignon up to Dijon to Mulhouse then to Basel Switzerland, then 2 hours to Lauterbrunen.
There are essentially 3-4 paths from Provence to Lauterbrunen. I think the high speed trains in France and Switzerland make that route best. You can actually drive from Nice to Italy, then into Switzerland, or you can drive from Lyon to Annecy to Chamonix, then into Switzerland via Sion. However, any driving into Switzerland requires a Swiss auto license, and would be a slow drive.
The last option is Paris to Lauternbrunen in a long day, then go all the way back to Paris then down to Avignon and spend a few days, then back to Paris. I think this is pushing it a bit though.