Like YuLaw? said, geographically SE Asian (even this is debateable - see Annamite Mountains below), but culturally East Asian:
- At a number of points in history, China occupied parts of northern Vietnam, so the Vietnamese picked up much more Chinese culture than did the Lao, the Thai or the Khmer (Cambodians).
- Vietnam, like China, Korea, and, Tokugawa-era Japan instituted Confucian based exam systems for its bureaucrats.
- Vietnamese Buddhism is also more often Mahayana, again like the Northeast Asian countries, as opposed to Theravada, like Laos, Thailand, and Cambodia.
- Prior to the French introducing a Latin based script, Vietnam used Chinese characters, something the other SE Asian nations never did.
Historically, South Vietnam has closer ties to Cambodia than does the North, and was at one time an entirely separate nation, Champa which had more Hindu, Theravada Buddhist, and Islamic elements than Vietnam. How much of that remains, I don't know. There are still some Muslim Chams in the south, but not many.
Laos is very closely related to Thailand. Their languages are very similar, and Northeast Thailand's regional dialect, Isan, is essentially the same language as that spoken in Laos. Culturally, there is little affinity between Laos and Vietnam. Crossing the Annamite Mountains, which separate the two countries, is like crossing into a different dimension: the language, the people, the weather, the flora and fauna all change drastically.
Cambodia is its own entity, though it bears more in common with the Lao and Thai than with the Vietnamese - they are also Theravada Buddhists. Other than that, I don't know much about Cambodia.
Hope that helps.
