It is not easy to isolate the histories and cultures of Vietnam and Cambodia from the rest of Southeast Asia, or even greater East Asia. These countries have all influenced each other, sometimes forcefully, across the ages. The article will focus on those influences that have had an impact on these two countries, though you will see lots of overlap. For a more thorough discussion of the whole region, see the Ocean Cruise section on Southeast Asia.
The Mekong is one of several important rivers that have flowed for hundreds of years out of the eastern Himalayas. Each river valley was the birthplace of a nation:
Yellow (Northern China)
Yangtze (Central China)
Pearl (Southern China)
Hong, or Red (North Vietnam)
Mekong (Khmer: South Vietnam and Cambodia)
Chao Phraya (Ayuthaya: Thailand)
Irrawaddy (Pyu/Pagan/Taungoo: Myanmar)
Brahmaputra (Bangladesh)
Over time, political events would blur these distinctions. The three China’s would unite. The Khmer, the Ayuthaya, the Taungoo, and the Vietnamese would each take their turn dominating the Southeast Asian region. The Vietnamese would appropriate the southern Mekong Delta from the Khmer. Otherwise, the nations of today adhere pretty closely to their original river valleys. When a Khmer prince wanted to separate himself from the central Khmer influence, he moved further up river on the Mekong, to another valley and formed Laos. The two countries have always been tied together culturally.