Found on Jonas' body in the lab during Escape! at the beginning of the game. It is a message from your father explaining that he has reasons for leaving the vault and for you to not follow. This message was probably recorded moments before he and Jonas unsealed the Vault 101 door and your father left.
I thought about it for a long time, but in the end I decided it was best for you not to know. So many things could have gone wrong, and there's really no telling how the Overseer will react when he finds out. It's best if he can blame everything on me. Obviously, you already know that I'm gone. It was something I needed to do. You're an adult now. You're ready to be on your own. Maybe some day, things will change and we can see each other again. I can't tell you why I left or where I'm going. I don't want you to follow me. God knows life in the Vault isn't perfect, but at least you'll be safe. Just knowing that will be enough to keep me going.