Vault Number...
87
Starting Construction Date...
May 2066
Ending Construction Date...
May 2071
Now http://fallout.wikia.com/wiki/FEV_research on the other-hand:
2076
Unforeseen side effects began surfacing in early 2076 with the PVP. Animal test subjects began showing an abnormal growth rate accompanied by increased brain activity. The U.S. government took notice of these discoveries, and in the interests of national security, moved a team on-site to secure and oversee the project, which was now dubbed the FEV (Forced Evolutionary Virus) project.
So let's get this straight, FEV is created in 2076, later that same year the U.S. Government begins sending in it's own people to over-see the project in "early" 2076 - so perhaps March/April latest. The next log says that in 2077 they moved the entire project into the Mariposa MIlitary Base. So in the intervening months the Mariposa Military Base was designed.
Where in that chain-of-events does the U.S. Government remove the project from West-Tek, whilst the Mariposa Base is being constructed, then hand it to Vault-Tec with the orders to completely retro-fit a Vault finished half-a-decade before-hand to house this venture? Baring in mind that the U.S. Govt removed it to keep the Project in-house, if they were just going to give it back to another company what's the point? West-Tek are the most-advanced and largest military contractor and actually made the virus itself.
That's not even the end of it either, going by terminal logs in V87:
The latest subjects in the Evolutionary Experimentation Program (EEP) are showing some promise after only a single exposure to the modified FEV. We are currently testing five subjects, two males and three females. Each one of them is under 24 hour observation as usual. We hope to have a breakthrough in this strain as the continual pressure from Vault-Tec and the military at Mariposa is becoming most bothersome.
So these experiments where going on before the bombs had even dropped. Two scenarios, either they were just using the lab facilities for experimentation or they closed the Vault before the bombs. In both cases, why when the U.S. Government had Mariposa built specifically for FEV testing would any experimentation be going on elsewhere. And why is Vault-Tec involved when, again, the U.S. Government was keeping it out of anyone but the military's hands at their own secret installation; the point was to keep the project secure and insular, not fan it out amongst facilities on opposite sides of the country with other interests involved.
The Vault doesn't make sense, period.
EDIT: Cannibal, if you read this and start writing about how much all the Vaults don't make sense don't bother. This is in the Fallout 3 section discussing Fallout 3; it's not the battle-ground for pointing out flaws in the originals.