Unmissable story based achievement. Nothing difficult.
You'll be rescuing Vito Scaletta, the protagonist of Mafia II from some of Marcano's goons. Head into the restaurant and take cover. Wipe out the guys holding Vito, then bust open the lock to free Vito and trigger a cutscene.
Vito agrees to throw in with Lincoln's fledgling mafia family and you earn yourself another achievement.
(Video credit goes to Generic Gaming)