Facebook CEO Mark Zuckerberg and his girl friend Pricilla Chan spent Christmas Eve on a cruise ship on Ha Long Bay.
They were spotted first at a cell phone shop in Hanoi's Metropole Hotel on December 23 and the information quickly spread on the Internet.
Local news media on Friday quoted witnesses as saying that they saw Zuckerberg and Chan on Thursday visiting the Hoa Lo Prison Museum, known as the "Hanoi Hilton," where American prisoners of war were held during the Vietnam War.
A source told Thanh Nien that the couple departed from Hanoi's Gia Lam Airport in the morning on December 24 on two EC 155-B1 helicopters hired from the Northern Vietnam Helicopter Company.
Mark and his friends flew in one helicopter while the other was for their two guards, it said, adding that the payment for the two helicopters was about US$20,000.
A Quang Ninh tourism official told Thanh Nien on condition of anonymity that one of the helicopters landed on a hill at Bai Chay.
Zuckerberg and his entourage of eight people hired the Phoenix cruise ship and left the mainland at around 11:30 p.m. to cruise Bo Nau and Sung Sot caves in Ha Long Bay, another source told Thanh Nien.
Mark and his entourage spent the night in the bay and also rowed kayaks there.
On December 25, they visited other places in the bay before departing for Sa Pa by helicopter.
Earlier, a representative of Facebook in Vietnam confirmed that Zuckerberg and his girlfriend are in Vietnam for the Christmas holiday, according to local news website VietNamNet.
The report quoted Huynh Kim Tuoc, Growth Manager of Facebook in Vietnam, as saying that the company has confirmed that "Zuckerberg has arrived in Hanoi on December 12."
His trip is "personal and not related whatsoever to his work at Facebook," the report quoted Tuoc as saying.
According to comScore, Vietnam is a one of only seven markets where Facebook does not have the largest audience in the social networking category. The other markets are Brazil, China, Japan, Poland, Russia and South Korea.
Zuckerberg and Chan also made a low profile trip to China and Thailand for last year's Christmas holiday.