Most of the new cases were detected in Hanoi, at 2,884.
The capital's first Omicron community transmission case was a hotel employee who has made contact with foreign entrants infected with the variant, said Vu Cao Cuong, deputy director of the Hanoi Department of Health.
The employee has been quarantined at the hotel and there's no risk of further infection in the community, he added, without giving further details.
In the past 24 hours, 155 Covid-19 patients have died, including 19 in Hanoi.
Since the pandemic started last year, the death toll in Vietnam has climbed to 37,165, or 1.7 percent of all infections.
In the latest wave that hit the country in late April, 2.16 million cases have been confirmed and 1.92 million of them have recovered.
So far, 76.7 percent of the population have received two vaccine doses.