Excerpts from the book were published in Salon ahead of its 15 October release. Mr Osius alleges Trump made the remark ahead of his meeting with Prime Minister Nguyen Xuân Phúc.
"What's his name?" the Republican asked.
"Nguyen Xuân Phúc", a senior National Security Council official said. "Rhymes with 'book'".
"You mean like Fook You?" President Trump asked. "I knew a guy named Fook You. Really. I rented him a restaurant. When he picked up the phone, he answered 'Fook You'. His business went badly. People didn't like that. He lost the restaurant”, Trump said, as cited in the book.
The former official writes that all those present in the room "laughed dutifully". Mr Osius then proceeds to describe the negotiations between Trump and his aides that reportedly took place.
The Republican asked then-US Trade Representative Robert Lighthizer to “bring the US trade deficit with Vietnam to zero in four years” after he learned that Hanoi had a [$32 billion] trade surplus with Washington.
"Lighthizer nodded, perhaps not knowing how to reply. It was an impossible task. He then tried to shift the president's focus", Osius writes.
The former ambassador also described his conversation with former senior adviser to the president Jared Kushner. The two discussed a deal on the construction of a new US embassy in Vietnam. Kushner seemed surprised when he heard that the building could cost $500 million to build.
"Why are we spending so much? If we're going to give them that, we should get something back", Osius cites the ex-adviser as saying. "I wondered if he understood that we were trying to build a new embassy for the United States and not for Vietnam. ‘Our current leased space is dilapidated', I told him". "It was supposed to be temporary twenty-two years ago. It's not safe. A truck bomb could drive right up to it and blow us up in a moment. Like in Benghazi”, the ex-ambassador wrote.
Ted Osius resigned in November 2017 in protest against the Trump administration’s proposal to deport Vietnamese refugees back to their home country.