Vietnamese bank shares have outstripped the country's benchmark stock index this year as overseas investors plough into the market ahead of an expected liberalization of foreign ownership in financial institutions.

State-controlled Vietcombank on Tuesday dethroned PetroVietNam Gas as Vietnam's top listed firm, with a market value of $5.6 billion after soaring 44 percent since the start of the year.

Simultaneously, BIDV has jumped 60 percent and VietinBank has surged 37.4 percent. In contrast, the VN Index has risen just 4.75 percent.

Vietnam's main exchange data shows that 36 percent of foreigners' net stock purchases in May were in the top four banks, unchanged from April and higher than March's 19 percent.

Japan's Mizuho Corporate Bank owns 15 percent of Vietcombank, and Bank of Tokyo-Mitsubishi UFJ has a 19.73 percent holding in VietinBank. BIDV plans to sell a 25 percent stake to foreign strategic partners.

Investors, many from overseas with a long-term outlook, hope the government would soon ease a 30-percent foreign stake limit in local banks as promised by Vietnam's prime minister in April.

Higher share prices would be a boon to the government as it divests from banks.

Major stakeholders are also under pressure by the central bank to trim their holdings before the end of 2015 to previously specified limits.

Investors expect loan books to improve on stronger credit demand and a fall-off in bad debts mostly related to the once-frozen property sector.

Non-performing loans fell to 3.59 percent of total lending as of February from 17 percent in 2012.

But the rapidly appreciating bank shares have prompted concerns about relying too much on foreign investors.

"Stock prices have run at a much faster rate than banks' fundamental changes," said Hoang Viet Phuong, a deputy managing director at Vietnam's top brokerage Saigon Securities Incorp.

"This rally owes mostly to foreign purchases while domestic funds just follow suit. What will happen when foreigners stop buying?" ($1=21,790 dong)