In reply to Fredt:
Spent a very enjoyable week following the tour through the Alps and to Paris in a van this summer. Before this I would have said you would want to be there a week early to get a van spot but we managed to get pretty decent spots the night before on the Lauteret, Iseran (actually just before the climb as no camping is allowed) and Val Thorens. However, having ridden the Grand Columbier a couple of years ago I recall it is quite narrow (at least on one side) so finding a van space may be tricky.
I would recommend parking at the bottom* and riding up on the morning, this will make it easier to find a spot and to get away afterwards. Make sure you know what time the road closures are and which routes they affect. We had a tricky journey from the Lauteret to the Iseran due to a landslide closing a road forcing us to follow the tour over the Galibier and through Valloire meaning we had to wait a couple of hours for roads to reopen. It was worse the next day when the big landslide hit!
*or if the stage finishes at the top park on the other side and walk/ride over the summit.