Nyarlathotep...the crawling chaos...I am the last...I will tell the audient void.
Remember how the Vault Dweller writes memoirs or somesuch. Well, you could have a narrative looking back at how he saved and was cast out of his vault.
Also, the movie would have to lean really heavily on the uniquely Fallout retro-50s look, both to add comedy and distinguish it from other post-apocalyptic films.
Action scenes should be kept more grounded, rather than the current trend towards kismet-driven, dual-wielding, balls-to-the-wall stupidity that modern Hollywood leans to.