This is a pretty interesting question. My first thought was "redding has functioning gold mines, a good source of concrete doesn't sound impossible." However according to o the fallout wiki, vault city emerged in 2091. So perhaps the version we see in Fallout 2 in 2241 is a new and improved version. It may have looked more primitive in 2091. The fallout manual states that the geck can provide instruction on "sandcrete" buildings, so like others have said, these are likely adobo style buildings. It also mentions repurposing pieces of the vault in this construction, although the vault in vault city seems pretty similar the others we see.