Police in the central resort town Nha Trang summoned a taxi driver on Friday after a group of Chinese tourists accused him of overcharging.
Le Trong Quat, 40, drove the visitors to their hotel earlier this week.
As they arrived after 6 kilometers, his meter read VND62,000 ($2.73).
But he took out his phone and type a seven-digit number, the tourists said.
They said they had argued but the driver threatened them, and they had to pay him VND6 million ($264).
Police said Quat is under criminal probe for extortion of property.
Overcharing incidents like this one happen once in a while in Vietnam, earning the industry a bad name.
Last month, a taxi driver in Da Nang was fined after charging a South Korean tourist $30 for a $2 journey.