Pham Thi Thu, who runs an air ticketing agency in Hanoi, said only business class tickets are available for Hanoi-Con Dao flights but these cost VND7.5-8 million.
Economy tickets had been sold for around VND3.5-4.2 million, she said.
Hoang, owner of a travel agency in HCMC, said it is hard for tour companies offering Con Dao tour packages to find air tickets for March and April.
The most expensive Hanoi - Con Dao round-trip was only at VND5-6 million at this time last year, but is now 30% higher at VND6-8 million.
Travel agencies fear flights to Con Dao will become even more scarce and expensive following Bamboo Airways' suspension of services to the island from April.
The number of visitors to the island has steadily increased in recent years, according to Huynh Trung Son, vice chairman of the Con Dao District People's Committee.
Earlier this month he called on Vietnam Airlines to increase flights to meet the growing demand.
Heeding this request, the airline announced Monday it would increase the number of flights by 50% to 26-30 daily from mid-March.
Con Dao Airport can only handle a maximum of 33 a day.
Last year 72% of visitors arrived in Con Dao by air.