I just leared that I could have saved some people in Vault 34, I thought they were just feral ghouls but they weren't.
Thus I chose to help the NCR but now I really regret it. Is there any way I can still help those people in Vault 34?

It's only one person if I remember correctly, and it's a ghoul, slightly feral... incapable of normal speech. You gotta pick one or the other :/ and you chose NCR

Damn that's too bad... Where is he located in the vault?

you can't find him in the vault, if you transfer the vault controls over to the remote console he will release himself, you will then find him at the aerotech building

you can only do one or the other, save him(and his implied friends) or save the sharecroppers water(shutting down the vents and reactor)

Okay thanks for the info, guess I'll have to try that the next walkthrough.