I played Fallout 1, 2 and 3 and I don't remember if the fact that the vaults were designed to conduct social experiments was cited prior to Fallout 3.
As example, if you go look on Internet you will often see that Vault 12 (aka the Necropolis Vault) was designed with a voluntary problem in the front door so it was not fully resistant to radiations. This later transformed the inhabitants into ghouls. I think I found a note in Fallout 3 speaking about this but is there some reference to this fact in Fallout 1 and 2?
They may not seem important but if there is no reference to these social experiments in the first two games it could mean they were not really intended by the original creators of the franchise in Black Isles Studios and were instead introduced by Bethesda.
Best Answer
The idea of using the Vaults as social experiments was created by Tim Cain and was touched upon in Fallout 2. While reading the Vault 8 records, you can unlock a special file with a successful science roll. It shows that the vault was a'Control Vault'. Designed to remain sealed for 10 years then begin to re-colonise the surface. This site has some good information under the 'Vault System' section.
Here is a fallout wiki link to the known vaults, what game they were encountered in and the reason for their creation.