Not, correct!
Vietnam geographically lies in SE Asia, but Vietnamese feel belonging to East Asia. That is an ocean between Vietnamese and other people in South East Asia such as Cambodia, Laos, Thailand, etc...not to mention Malaysia, Indonesia.
Vietnamese cultures and custom are unique and first and foremost influenced by China, the impact of its other neighbors is just a little. You may find their SEA trace in food ort clothing. Besides China, the French had also a big impact on Vietnam´s cultures and custom. And of course the Americans too due to the conflict - the american way of life.
Well, some history books say the Viets were part of 100 Baiyue tribes. But unlike the other 99 tribes which were incorporated into Chinese empire, the Viets left ancient China land, moved South and established Vietnam.