The role of domestic refineries is "unknown in the gasoline supply equation," he told lawmakers during a Q&A session at the National Assembly Wednesday.
When Vietnam did not have a single refinery, there was no shortage, and the same goes for some other neighboring countries that do not have refineries, he said.
The Binh Son (also known as Dung Quat) Refinery has been operating steadily and meeting 30-35 percent of domestic demand, he said.
But Nghi Son, owned by foreign companies, has not been operating efficiently, he said.
The plant's biggest issue is finance, and Petrovietnam, which owns a 25.1 percent stake in it, is urging the other two main shareholders, Japan's Idemitsu and Kuwait Petroleum International, to fulfill their commitments to supply to the local market, he said.
Only when PetroVietnam guaranteed that the plant could operate normally would the trade ministry stop importing gasoline and diesel, he said.
The trade ministry recently instructed 10 gasoline distributors to increase their imports by 2.4 million cubic meters in the second quarter.
Supply has been erratic in the last two months partly because Nghi Son has had to reduce production from 105 percent to 80 percent and then 55 percent.
With the Russia-Ukraine crisis causing oil prices to spiral up, Vietnamese gasoline prices have risen by 28 percent this year to an all-time high of VND29,820 ($1.30).
Dien said the domestic increase has been less than the hike in global rates thanks to the stabilization fund. But since the fund is dwindling, authorities are considering reducing taxes, he added.