Do Vietnamese know Chinese and speak Mandarin? Is there a city called Ho Chi Minh City in Vietnam?

There is a city in Vietnam called Ho Chi Minh City. Yes

Ho Chi Minh City (Vietnamese: Thành ph? h？ Chí Minh//Chengpu Ho Chi Minh), formerly known as "Saigon" (Vietnamese: sà i gê n/Chaigan), is the largest city in Vietnam and one of the five municipalities directly under the Central Government. The present name of the city is to commemorate Ho Chi Minh, the founding leader of People's Republic of China (PRC), chairman of the Workers' Party of Vietnam and chairman of the Democratic Republic of Vietnam.

The metropolitan area includes Ho Chi Minh City, Toulomu (Pingyang Province), Di 'an (Pingyang Province), Bianhe City (Tongnai Province) and surrounding towns, with a population of over 9 million. It is the largest metropolitan area in Viet Nam and the former French Indian zhina. In the metropolitan area of Ho Chi Minh City, the metropolitan area covers most areas. Qianjiang and Longan provinces in the southeast are planning a city of 30 square kilometers and a population of 200,000. According to the prediction of Mercer Human Resources Consulting, Economist Intelligence Unit and ECA International, by 2020, Ho Chi Minh City will become the most expensive city in the world.

Vietnamese who haven't studied Chinese certainly don't know Mandarin.