It was collected "from a popular school website in Vietnam" in July, the unidentified hacker, who goes by the name meli0das, posted in an online forum last Friday.
"The database is very useful [...] because 30M is nearly 1/3 of the Vietnamese population," the hacker said.
meli0das, who joined the forum last month, wants the $3,500 in Monero, a cryptocurrency with privacy-enhancing technologies to increase anonymity.
Each person’s record includes full name, email, phone number, birthday, grades, school, and location, the hacker added, saying the database "has never been leaked before."
VnExpress found accurate a sample of 70 records the hacker released to prove the authenticity of the data, but could not verify the scale of the leak.
A security expert has tried to contact the hacker, but said the hacker was "pretty cautious."
"In previous leaks, hackers were willing to provide samples. But meli0das only provided samples to trusted accounts on the forum or those who proved to have enough money."
The hacker is also offering data about "360,000 Vietnamese students" collected from an educational website.
Ngo Minh Hieu, a security expert at the National Cybersecurity Center, said there is a high chance that the hack has happened, but is skeptical about its scale.
"If true, this will be one of the largest data breaches in Vietnam."
Last week another hacker, who goes by the name ChinaDan, offered on the same forum what is described as data on a billion Chinese citizens for $200,000.