The "Vallée blanche" you mentioned is not very far from "Val d'Arly" or "Val Thorens" in the Alps. Both words "val" and "vallée", and also "vallon", come from the Latin "vallis". See the Littré dictionary https://www.littre.org/definition/val for examples over the last millennium.

By the way "Le dormeur du val" (The sleeper of the small valley) is one of Rimbaud's most famous poems, often learned at school.