PossibleCabbage
Vault 22 Survivor
I don't think I would ever like to play prequel in the Fallout world, Ever since inception Fallout has always been moving forward , It's more fun to see the whole picture mature.
I think the important thing about "Fallout" as a franchise is that you're talking about what comes after the nuclear apocalypse (the "fallout" from it, as it were) rather than what came before.
So a "prequel" set more or less any time after October 23 2077 could be fitting with what Fallout is. After all, different parts of the world would be targeted differently in a nuclear exchange, and would develop differently afterwards because of different local resources. So there's not that much difference between "this place got hit hard and didn't have a lot of things around to help it recover" and "this game takes place in 2100" in terms of the level of progress people have made.
You don't even need to reference the date if you set a Fallout game in a new place, really. If you wanted to set a Fallout game in Oklahoma, they could just have their own calendars and the only place the timeline would matter is "what elements from outside of here have influenced local society."