Technically that counts as the correct answer and because the game's been going on for a long time, I'm going to give the

to you. The exact answer was the basement under the Tops, where Benny flees if you let him go. The place is connected to Vault 21, however, and that's where you end up when chasing him. You can't actually catch him, though.
From the wiki:
The sub-basementThe sub-basement itself is a small room with a breached wall leading into a sealed-off section of Vault 21. This section is presumably part of the reactor maintenance section for the vault. It does however contain some minor loot in lockers and several explosives crates that Benny may have used in excavating the section.
If one explores the unfinished vault, they will find several inaccessible doors, but there is one door at the end of a corridor that, if opened with the console command "unlock," will lead to a void. If one jumps inside the void they will be teleported to the real Vault 21. Then they can simply turn around and leave to New Vegas or explore the vault.