Vietnam is recognised as a country with its own territory, people and history, just likes any other country recognised and bounded by all members of the United Nation, does not matter if it has short or long history.
Vietnamese people are those who permanently live in Vietnam as legally Vietnamese citizens, or those who decide to be Vietnamese regardless they live in any other country who came from Vietnam or whose ancestries were from Vietnam, and regardless they are from any Vietnamese ethnic background.
I am an Australian citizen but born and grew up in Vietnam, I consider myself an Australian and a Vietnamese. I have many friend who are Australian citizens with Vietnamese-born Chinese ethnic backgrounds, they call themselves Australian and also Vietnamese.
So, there are two angles to be Vietnamese people. The legal angle and the emotional angle.