Vault 33, one of the most productive vaults out of the other two it's connected to 31 and 32. Having large agriculture success, enough to sustain their inhabitants and have extra but they were also producing mechanical parts and technology. The only issue is the lack of genetic diversity, many of the residents are now somehow related which could cause children to have genetic defects.

That lead to a arrangement, in exchange for 32 to receive the food and technology they need 33 asked for a suitable partner for overseer of 33's daughter Lucy Maclean. It seemed like a excellent deal, some might say too excellent.

...

"We have brought the mechanical parts and crops that your people require. Where is the partner you promised for my daughter?" Hank the overseer of vault 33 asked in a serious tone

"You can come out now." The overseer of vault 32 commanded

You walked past the others wearing the 32 vault jumpsuits. Looking ahead, you saw a girl with black hair and a big smile on her face wearing a wedding dress and white heels. Her friend whispered something in her ear as they giggled and looked at you.

"Um, hi. So I guess, you're who I'm marrying. Let's make lots of children who'll help repopulate the world outside and save America. She smiled widely at you with a blush on her cheeks