Fallout – What benefits do I gain by building terminals

From my experience so far, I'm going to say Starlight Drive In. This is because of the following:

• It has direct access to water for water purification (the middle of the parking lot has sunk and has radioactive water)
• It has a lot of land, with some very good vantage points from the diner at one end of the parking lot and the sign from the other end. Plenty of locations with excellent visibility to install turrets, the rest of the parking lot is quite flat
• It's relatively centralized to the initial areas (half way between Cambridge and Sanctuary)
• It has enough dirt to plant food to fully maximize your population without any issues
• There is a lot of scrap (old cars etc)

The Fallout wiki has an extensive list of known Vaults, as well as their purpose. Each vault description is backed by in-game material you are suppose to look for, but it does appear to confirm that there are no other vaults to be found as of yet.

• Vault 75: Vault 75 is an experiment in improving the human genome. It appears these tests were carried out using selective breeding, genetic modification and hormonal treatment. The test subjects have their growth accelerated, and at 18, are "disposed of". It is suggested that in certain situations, such as for the purpose of replacing research staff, exceptional candidates may have been used for other means.

• Vault 81: Vault 81 is a testing facility involving antibodies, disease and radiation. Vault 81 was under express orders to not evacuate under any circumstances, unless an official "all clear" was issued. In the event of an evacuation, it is suggested that the overseer would order the 'dweller section' to be mass-incinerated, including the dwellers.

• Vault 95: Vault 95 is a social experiment on isolation and drug addiction. All dwellers were drug addicts, and the experiment started pre-war.

• Vault 111: This is the vault our hero comes from, and was a test in cryogenics and long-term suspended animation.

• Vault 114: Vault 114 was another social experiment, where the inhabitants consisted almost entirely of upper-class society. The living conditions of the vault was advertised as highly luxorous, but in reality, it was the exact opposite. The overseer was chosen from the general population, and the interview process favored qualities such as no leadership skill and issues with authority. The purpose is listed in a terminal; "By taking away the luxury and authority these groups saw in surface life, we hope to study their reactions in stressful situations."