When a U.S citizen coming back to U.S after traveling?

I've heard a case that a Vietnamese, who became a U.S citizen, came to visit Vietnam and stayed in Vietnam for a year or more (he had extented his vietnamese visas many times). Then when he came back to America, he had troubles with the U.S Immigrant Officer at the airport and the Officer didn't let him in with the reason that he had been living in another country for too long. Does anyone know what happened in this case? If we visit Viet nam for long (like 2 or 3 years with extented vietnamese visas), do we have to notice the U.S Embassy or something like that?