If you are in Hanoi as a diplomat, you may import your private car from your home country. Your embassy will use official channels to arrange the shipping and customs registration for your car.
Individuals working independently or for private companies are not legally allowed to import private vehicles into Vietnam. This law is designed to protect local car dealerships, as well as government revenue from taxes–all cars imported to Vietnam for retail sale are subject to at least 200% tax, and for luxury vehicles taxes can amount to over 300%.
However, laws change, so if you wish to import your car to Vietnam you can periodically check the Vietnam Customs official website for current regulations. The website deals primarily with wholesale imports and should not be considered definitive or reliable, but you may contact Vietnam Customs through this English-language website: http://www.customs.gov.vn/English/Default.aspx
Some foreign individuals have imported private vehicles to Vietnam through bribery, but this is a criminal act, involving organized crime and corrupt officials, and is extremely expensive.