I used to live in Chamonix. I drove a van thorughout the winter and never had any problems on the main roads. Even when the snow is extremely bad, they keep the main roads in a fantastic state. The A40 is dual carriageway from Geneva and rarely has snow at that altitude. Once your start the climb, the gritted main road (E25 will be fine, just go steady on the 180 bends)
I hope your not too claustrophobic for the 11k tunnel!!!!
There are blue markers in the tunnel to keep distance between vehicles, make sure you do, they are very funny about it....
I hope the weather is clear for your trip, so you can see your valley in all its glory!!!
Adventure is worthwhile in itself.....
Riding a Russian sidecar to the Arctic Circle www.3wheelsnorth.com