Hey friend! First of all thanks a bunch for your support, I hope you and your players will have a blast
Normally I'm not making keys for my maps because I'm letting GMs free to use their imagination without limitations. However, since you asked I can describe from my own games:
1. At the south east corner, there's an elevator which raiders couldn't hack and open. Therefore, the whole underlevel is untouched and still functioning. However, after the upper floors were raided the Seer got a breakdown and after initiating the lockdown he killed himself, locking everything out. Unfortunately the people at the underlevel also couldn't exit and they died due to thirst.
2. Vault 74 was experimenting with immortality through chemicals and blood transfusion. So the 4 rooms at the south - south west are the laboratories where the R&D team was researching the chemicals and doing actual experiments. This is also the reason the elevator had super high security. Each lab from right to left has a different security level. The one at the most right has the lowest while the most left one has the highest clearance level. As you can see from the door lights, the only one open is also the most right one. All others require some good hacking or other skills to enter.
3. In my game, the players must enter each one of them and reach research notes in order to figure out how they can brew this experimental chemical. However when they enter the last room, they're gonna realize one of the workers locked down their computer (red light) and the only way to get into it is a super high hacking skill. If no one could, then they're gonna make only 1 dose of experimental drug because that locked computer was actually needed to learn how to duplicate the serum.
4. In my game experimental serum will grant 1 permanent Endurance SPECIAL if used on self. If they manage to hack the last computer, they're going to be able to make up to 4 doses (or how many players are in the game). Each serum can be used only once and if you use the 2nd dose, it will make you lose 1 permanent charisma because it will turn your skin greenish color; without giving anything extra.
5. If they can manage to enter the Seer's office, they can try to lockpick the safe behind the desk where all 4 scientists' access cards were kept. With these access cards they don't have to hack computers or doors. They open everything.
6. The room with glowing mushrooms has the containers that are required to brew the experimental brew. However the room is extremely radioactive. If they think there's an unlimited amount, nope. 99.9% of the ingredients are expired hence the high radiation because some of the nuclear materials even melted down. So they will find ingredients here enough for 1 dose only (which they can duplicate with a special technique)
7. The last room at the west is the Fusion Reactor. It's still functioning but due to lack of maintenance for 100s years it protected itself to work in backup mode, providing minimal energy enough for only life support units and computers.
I hope this will give you some inspiration. Sorry for a very short story but I don't want to spoil your imagination.
Take care!