It will check their operations, permits, facilities, and imports since early last year. The investigation is expected to take 70 days.
Three jet fuel suppliers will not be scrutinized.
Gas shortages are still reported at some stations in Hanoi and HCMC though retail prices have been hiked to an eight-year high.
The shortage began earlier this month due to a cut in production by Vietnam’s biggest refinery, Nghi Son, though officials have said repeatedly supply is adequate to fully meet demand in February.