Well, the Vault(Fallout Wiki) helps a lot. But, I think you can gain the most amount of info by playing the games themselves.
Ive played all 3 games (4 if you count fallout tactics, which wasnt part of the fallout series i believe)
I have completed them (all be it, many moons ago) started back on Fallout 3 again, had a wee break from it. But having played the game, i dont seem (maybe cause im a thicko) the in-depth knowleadge of the game.. I know major facts, 2077 Great war happned, lasted 2 hours, China vs USA etc... but i know the Enclave, appear to be the evil side and BOS seem the Good side, but didnt reliase there was a true deep history/long run storyline for these sides.
I have read (seemingly) quickly through the Fallout Wiki page and have now just downloaded the Fallout bible..
Maybe i need to "Explore" and "Read Into Fallout Lore" alot more to understand the true gameplay generics and history..