The Oil Rig was destoryed in 2242, you fight the Enclave in FO3 in 2277. That's 35 years. It wouldn't be difficult to double your population in that time. Additionally it was implied even in Fallout 2 the Enclave may have had other basis across the wasteland. How else could the Chosen One infiltrate Navarro as a new enclave recruit? And the Enclave defector A. Ron Meyers had never been to the Oil Rig. So the Enclave may have established colonies dotted across the U.S to maintain a sufficent population.
We could also assume that releasing the virus onto the Oil Rig is non-cannon and perhaps most of the citizens managed to evacuate.
All the Vaults were opened but for Vault 13 and then Bethesda went a ripped off that and made Vault 101.
And you base this completely on your own hatred for fallout 3 and not on anything that was actually in the game material. Vault 13 was intended to stay closed for two hundred years to test long term isolation. Its not out of the question to have a vault designed to test how long could it possibly surrive before failing. Two important facts, The creators of fallout intentionally left most of the vaults undesigned for future games and even fanfiction. So to say "all the vaults" is just plain ignorant.
And secondly Vault 3 (in NV) was sealed for a couple hundred years until a water leak forced them to open up for trade.(which led to there destruction at the hands of the fiends.) How do you explain the original creators of the series adding this vault which remained sealed longer then any vault so far? Keep in mind that the original creators of the fallout series MADE New Vegas.
People talk of Vaults as nothing more then a Myth or legend
No they talk about Vault 13 as a myth because the cannonized hero The Vault Dweller came from there.
Raven Rock was the combat base of operations, still too much of a risk to have the women and little plasma shooter tykes. They had death claws and crazy stuff in there. Too much of a chance for some kind of disaster to happen. No place for their highest priority. Making more pure humans........
Vault 13 had a population of one thousand residents, yet you don't see the space for that many in FO1 do you? You need to learn about a little thing in RPG games about representative population and representive area. You only need to see enough for the PC to interact with.