The exact time for vaccination would depend on the Ministry of Health’s vaccine distribution for the city, with the priority to be given to children aged 16-17 first.

Hanoi to start Covid-19 vaccination for children in fourth quarter
If students return to schools as normal, vaccinations will be conducted at schools.
The Covid-19 vaccination will be administered first to areas with a high risk of Covid-19 infection such as those with new cases, high population density or located near localities with Covid-19 outbreaks.
According to Khong Minh Tuan, deputy director of the Hanoi Centre for Disease Control, around 700,000-850,000 children in the city aged between 12 and 17 would be eligible for the vaccination.