* Shares hit 39,200 dong by midday break vs 28,000 dong open price
* Volumes thin due to low free-float - trader
* Jump could boost govt stake divestment plan (Adds trader's comments, context)
By My Pham
HANOI, Jan 3 (Reuters) - Shares of Vietnam Airlines surged 40 percent on their stock market debut on Tuesday, valuing the firm at $2.1 billion, as investors sought exposure to the flagship carrier to tap into strong air travel growth in the Southeast Asian nation.
A strong stock market showing could prove beneficial to the Vietnamese government's stake divestment plan. The government wants to sell off its stake in several state firms, including brewers Sabeco and Habeco, and use stock market prices as benchmarks to value its holdings in them.
Vietnam Airlines shares hit 39,200 Vietnam dong ($1.72) at the midday break on the Unlisted Public Company Market compared to their opening price of 28,000 dong, data from the secondary exchange showed. There were bids for 1.3 million HVN shares, while just 700 shares were bought, the data showed.
The carrier is 86 percent owned by the government, with another 8.8 percent belonging to Japan's ANA Holdings, meaning very few shares are freely available for trading.
"There was limited supply due to the low free-float, while sellers were few as shareholders were waiting for higher prices," said Duong Manh Dung, a trader at VnDirect Securities.
Vietnam's airline market is growing at one of the fastest rates in Asia Pacific, boosted by a burgeoning middle class, with the country's airlines placing multi-billion dollar aircraft orders and the Southeast Asian nation unveiling plans to shore up infrastructure.
Vietnam Airlines said on Monday its 2016 pre-tax profit hit a record high of 2.5 trillion dong, jumping 140 percent from a year earlier.
It has a 70 percent stake in Vietnamese low-cost carrier Jetstar Pacific, with Australia's Qantas holding the rest. Its other domestic affiliate is Vietnam Air Service Company and it also owns 49 percent of Cambodia Angkor Air, a joint venture with the Cambodian government.
The airline commands about half of the domestic market while the two affiliates have another combined 15 percent share. Together, they announced deals last year to buy 20 Airbus jets worth an estimated $4.1 billion to expand their fleet.
The stock market debut of Vietnam Airlines comes as private sector rival VietJet is gearing up to make its market debut and has been valued at $1.2 billion, according to Thomson Reuters publication IFR.
Vietnam late last year also saw several major share sales and listings, including a $3.72 billion listing of its top brewer Sabeco where the government owns nearly 90 percent and is keen to unload its entire stake by 2017.
Other closely watched offerings were the listing of Habeco and the sale of a portion of state shares in dairy products maker Vinamilk.
Vietnam has been seeking to accelerate the initial public offerings (IPOs) of state-owned companies and list their shares to boost investment and increase transparency. ($1=22,750 dong) (Reporting by My Pham; Editing by Muralikumar Anantharaman)