Vietnam joins FTSE Russell emerging market benchmark

The transition will take place in four stages through 2027, with 10% added in September, an additional 20% in March, and 35% each in June and September of next year.

"As we get closer to the next inclusion tranche in March, attention should pick up again, and because the allocation will be larger, local investors are likely to see a more noticeable impact," Nguyen added.

Vietnam's benchmark stock index .VNI opened up 0.54% on Monday, led by banks, before retreating slightly.

Anticipation of the upgrade helped revive foreign interest in Vietnamese equities. According to data from the Ho Chi Minh Stock Exchange, overseas investors bought a net 2.7 trillion dong ($104 million) worth of shares last week, but they remain net sellers by about 91 trillion dong.

After the initial excitement, however, interest is likely to fade, said Thomas Nguyen, chief global markets officer at SSI Securities Corporation, the country's second-largest broker by market share.