Ho Chi Minh City is in the Indochina Time Zone (ICT.)
The offset to UTC (or the old GMT - Greenwich Meantime) is +7
So that means:
It's already 7:00 am here in Saigon when it turns midnight in the UK.
(And for reference - 11 am in Sydney, 7pm the night before in New York, and 4 pm the afternoon before in LA.)
It's the same timezone for the whole of Vietnam, and there is no daylight saving.
Other destinations in the same time zone include Cambodia, Laos, Thailand, Mongolia, and parts of Indonesia (Java, Sumatra and the Western part of Kalimantan).