It depends on the weather but early june at low-mid altitude in the Vercors is a bit late : Crocus, Erythronium, Gentiana, Corydalis and Orchids are already wilted. The peak is between second week of May and mid-June. End of March/early April is pretty fine for vernal flora. Early June is perfect for hiking in the Ecrins
PM with your email i'll give you some nice spots you can't miss.