Transiting involves more costs and time, which tour agencies need to factor into prices, automatically causing them to rise. This puts tour companies at a disadvantage in terms of bargaining power.
There are currently, for instance, no direct flights from Vietnam's capital city Hanoi to Turkey and major European destinations like Czech, Spain and Italy. There are also no direct flights from Vietnam to places like Sweden and the Democratic People's Republic of Korea (DPRK).
"A lack of direct flights can make visitors hesitate to travel, because it takes them too much time to transit," said Nguyen Cong Hoan, a representative of travel firm Hanoi Redtour.
An executive of a Hanoi-based travel company said the number of Vietnamese tour bookings to the DPRK has significantly increased in recent years, especially after the Trump-Kim summit in Hanoi last month.
"But there are no direct flights. Since tourists need to transit in China, the airfare often accounts for as high as 50-60 percent of the tour price," the source said, declining to be named.
"Sometimes when only one airline operates a flight to a certain destination, it makes the tour price uncompetitive [as a result of the monopoly]. And sometimes we have to split a tour group into two, because the plane does not have enough seats," the source added.
Truong Thu Giang of Vietravel, a leading travel firm in Vietnam, noted that some countries such as the UK, Canada and the US require tourists to have a visa even for transiting.
But things are changing, with an increasing number of direct flight routes beginning recently or being launched soon as both outbound and inbound demand surges.
Qatar Airways began direct flights to Da Nang last December, and the Republic of Korea’s Jeju Air followed suit with daily flights between Da Nang and Daegu city.
In early March the US allowed Vietnamese airlines to operate direct flights to its cities.
The state-run Vietnam Airlines said it is considering buying at least two new long-range Boeing jets in preparation for beginning services from Ho Chi Minh City to Los Angeles or San Francisco in California in 2020. Initially the flights would stop in Japan, Taiwan or the RoK before nonstop services begin in 2022, it said.
Vietnam Airlines is also eyeing the Vietnam-US West Coast route, which is estimated to have one million passengers a year, "the biggest unserved nonstop route," it said.
Private carrier Bamboo Airways early this month placed an order for 10 wide-body Boeing aircraft. The airline is preparing to launch flights to the US in late 2019 or early 2020, it said in a statement.
According to the Civil Aviation Authority of Vietnam, there are 68 carriers from 25 countries and territories and five domestic carriers flying in Vietnamese skies.
The country has become the world's fifth fastest growing aviation market, and is expected to reach 150 million passengers by 2035, according to the International Air Transport Association.