That's why it's called a developing market. Many people expect that they can buy an index fund of any country similar to how you buy it in the US or some other developed market and get a return in line with the country's GDP growth. Often it's not the case.

You can’t invest in much of Vietnam and its biggest businesses due to regulations banning foreign ownership.

Look into closed-end funds if you want to better approximate what a real index of Vietnamese businesses would look like.