If i had sex 7 days ago and i get hep b vaccine now or next week. would it prevent it?

No protection: Hepatitis B infection is rare after any single heterosexual exposure, unless your partner is at special risk (e.g. bisexual, injection drug user, etc). If your partner was another male, the risk is higher, but still not common after any single exposure. The vaccine takes a few weeks to become protective. If already exposed, the vaccine alone will not prevent infection.

Wouldn't work. : One dose of vaccine would not be that effective that quickly.