In reply to Derry:
I used snow shoes to access routes in Chamonix for several years in winter and spring before I learnt to ski. They do extend your range enormously, and make life much easier. You can rent them in the valley. If you arrive after a period of dry weather, some south facing rock routes will be in condition, including in the Vallee Blanche. Snow/mixed routes ditto. But this is prime time for steeper ice. The goulottes will be excellent. Easier, more accessible options would include the Col du Plan, the Mallory - Porter on the Midi, the Y couloir on the Aig. d'Argentiere. Maybe the Whymper or Couturier on the Verte, if you're confident.