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Yes, visa on arrival is NOT VALID for OVERLAND ENTRY. The four international airports in Vietnam that you can receive your visa stamp with this approval letter are: Noi Bai International Airport (Hanoi), Tan Son Nhat International Airport (Ho Chi Minh City), Cam Ranh International Airport (Nha Trang) and Danang Airport (Da Nang).
Note:
If you wish to travel to Vietnam via land or sea, you need to obtain your visa through Vietnam embassy in your country.