What i didn't understand is why Vault Tech created these experiments.
Vault Tec Didn't (really). The Pre-War "Enclave Conspiracy" did. Vault Tec is just one appendage of the Enclave Octopus.
Before the war started, a group of powerful and influential (not to mention wealthy) Americans looked at the geopolitical situation given the slow dwindling of resorces, etc. Their conclusion was that a Nuclear war in their lifetimes was not just likely - it was Inevitable and the likely result was an uninhabitable planet. However this nuclear holocaust need not go to waste - The good and the greatest could be saved, everyone else - well, they're not important.
The result of this planning was the "Enclave Facility" - The Oil Rig in FO2. Had Van Buren come to pass, this would have been coupled with a Spaceport facility for a plan to colonise another planet.
The effects of large groups of people in a confined space, such as a starship is not very well known, particularly over a period of generations (which is likely to be how long it was going to take to get to the nearest star system, Hyperdrive isn't invented yet) as such the "Enclave Conspiracy" decided that it would be a good idea to collect more information on the potential effects before blasting off. Enter "Project Safehouse", a series of 122 shelters (or "Vaults") each with a few unique challenges to attempt to determine what possible problems the Colony ship may have and how to avert them, and what the best approach would be to crewing this ship. This also had the additional "bonus" of giving the "unimportant" people a safety blanket.