Cancellations and delays affected between a quarter and half of the flights operated by Vietnam's three carriers.
Aviation authorities blamed the problems on poor crisis management skills.
News website VietNamNet cited a statement by the Civil Aviation Administration of Vietnam reported a 42.3 percent overall increase in flight delays and cancellations year on year.
That figure was 50 percent at low-cost carriers, including Jetstar Pacific and VietJetAir.
The CAAV said 14 percent of Vietnam Airlines flights were cancelled or delayed during the first five months this year compared to 17 percent of those provided by Vietnam Air Services Company in partnership with Vietnam Airlines.
The agency reported that, 50 percent of Jestar flights and 51 of VietJetAir flights suffered the same fate.
When late flights were canceled or delayed, many passengers were forced to spend the night at airports.
CAAV blamed the delays on technical problems, bird strikes and inclimate weather, staff and passenger problems, the carriers’ limited capacity to accommodate peak travel demand, and commercial reasons.
Photo credit: Nguoi Lao Dong
The agency noted that the delays set off a number of quarrels between customers and low-cost carriers for failing to provide satisfactory explanations or solutions. At other times, the website reported, the carriers didn't make staff available to answer questions.
The carriers were accused of being poor problem solvers, as they usually informed passengers of the delays or cancellation at the last possible minute, and repeatedly changed flight times.
CAAV Director Lai Xuan Thanh said his agency will publish flight delays and cancellation information starting in July.
Vietnamese carriers handled 16.3 million passengers in 2014, up 13 percent year on year, and 372,000 tons of cargo, up 24 percent, year on year.