All I can think of is that I remember a mod for Fallout New Vegas that was announced years ago that takes place in a completely different game world thats in-lore with the Fallout universe. Takes place in another region of the United States somewhere that we haven't seen before in past games?

Was it The Frontier?

https://www.falloutthefrontier.com/

Isn't that the really bad one because no I don't it was the Frontier I was thinking of.

New California?

New California?

Didn't know about New California, I'll take a look at it in the future.

https://new-california.fandom.com/wiki/Main_Page

I think that's it. Thanks.

So New California is the place where the first two games took place in right?

Does it feature Vault 13 or New Reno or The Hub?

I believe it was originally named Project Brazil and as far as I'm aware the action takes place initially in a vault in New California.
I haven't played it myself as I'm not that big into modifications for Fallout (save for jsawyer's mod for balancing and NVTF/nvse for stability, since Gamebryo is a dog of an engine).
I honestly doubt anything can recapture the atmosphere of the first two games or even come close. The first Fallout was one of the best games I've ever played, so it holds a special place in my heart. Dark, to the point, difficult at times, with an excellent soundtrack and good writing. But I digress...
And yes, California is at the core of the Fallout universe. Van Buren was supposed to expand on that and offer some interesting locations, shame the project got cancelled.
Anyway, New California might be worth a shot, maybe you will find it entertaining.

Are you sure it's not https://taleoftwowastelands.com/index.php

No it was New California that I was thinking of

I need to try New California, but have not heard good things. There's also one set in the Pacific Northwest, and another in Miami, that are hopefully completed someday.

The Miami one is for Fallout 4 though.

I played the first part, back when it was just project Brazil, and I loved it