In reply to filiperosa:
I would say that you need to be mega fit as there probably won't be a train to Nid d'Aigle, also the lift up Belle Vue won't be operational, therefore you'll be walking from the valley floor in les Houches. I went up there early July a few year ago and the tunnel into Nid d'aigle was still packed with snow, this year there is still a lot of snow around.
If your aptitude, finances and skills allow, I'd recommend the Midi telephrique and go via the Trois Monts route, and for that you will need crampons for part of it.