First off, the fundamental problem facing any game-to-movie adaptation is that so few games, save maybe action-shooters, can snugly fit the Hollywood Formats of Action, Drama, Horror, Romance-Comedy, et al.
Fallout as a concept has elements of almost all of those categories, from the drama of leaving the Vault, the adventure of going through some epic quest & fighting off raiders, horror in things like mindless brain-eating sewer ghouls (and ghouls in general) the macabre and murderous tendencies of Super Mutants, etc.
Honestly, the closest I could come to categorizing the ideal Fallout movie would be an Adventure/Survival Horror with lots of dark humor.
Sadly, I think doing it "right" as a live-action movie would be very difficult at best without a Spielburg budget.