Hi Tony, we did this tour a few years ago (Sep) and loved it. You are in for a treat!
There was quite a variation in the temps. We had a beautiful, sunny day in Chamonix. I was comfortable in a long sleeved T-shirt and ‘I ❤️ Paris’ sweatshirt on the Aiguille de Midi. After that we took the train to the glacier and ice cave in Montivilliers (buy the combo ticket). The gondola to Italy was closed for renovations. We loved Chamonix and didn’t want to leave. Fortunately, it was rainy, snowy and cold on departure day so that made it a bit easier. We boarded the bus and later that day arrived in Vaison-la-Romaine to 98 degrees… hot hot hot. I bought a pair of shorts.
If I were to do the trip again, I’d pack a lightweight puffer jacket. I have one that squishes into a pocket. Loved the itinerary, especially Beaune, Chamonix, and Vaison. Lots of good wine and as mentioned, the memorable lunch in Provence. Hope you have a great trip!