It should be noted that the 1000 years (closer to 900) was not continuous.

First occupation: 111BCE to 39AD

Second: 44 to 500s (ok so only a four years difference there)

Third: 602 to 905

Fourth: In the 1500s for about twenty years

I study archaeology, anthropology, and international studies. East and Southeast Asia happen to be my areas of study. So that is the angle I am coming from. Vietnamese people are ethnically unrelated to Chinese people. Native Vietnamese people (they are called Kinh) make up around 90% of Vietnam's population. Han Chinese people make up around 1% of the population. This alone shows that the Kinh (Vietnamese) have maintained a distinct ethnic identity for what is essentially 5000 years. The Kinh dominated the area for a couple millennia before the (Han) Chinese even interfered. They had already established a long ethnic and cultural tradition at that point. The Chinese did indeed hold onto Vietnam for a long time. They influenced a lot of Vietnamese culture. However, there was very little integration of the two groups of people. The Kinh held onto their ethnic identity as well as their distinct language (Vietnamese is an Austroasiatic language, while the Chinese languages are Sino-Tibetan; the languages are totally unrelated). This ethnic disparity is important when you consider Vietnamese identity. As far as this is concerned, they are totally separate. There are influences, sure, but that has not changed the Vietnamese identity. That identity has existed for almost 5000 years, really (about when the first Vietnamese states came about).

Consider Japanese history. Japan has been hugely influenced by China for millennia. However, that never interfered with what we probably perceive as a very distinct Japanese culture. The same can be said for the Vietnamese.

I realize that anecdotal evidence is probably discouraged, but in this case I'm using it as a bit of an example. One of my best friends is from Vietnam. He acknowledges the influence of the Chinese, but he does not see a relation with them. He identifies as Kinh, and he says that's how most Vietnamese that he knows feel (coincidentally, that is how the other Vietnamese people feel that I've talked about this subject to). On top of this, as was noted by others, the occupation was not constant. This is important as well. There were rebellions by the Kinh throughout the occupation. Evidently, a rather strong Vietnamese identity was present. They were rebelling against foreign people controlling them.

Conclusion: yes, they thought of themselves as different. In fact, they were and are objectively different, as they are not descended from the Chinese. Totally and completely different ethnic groups.

Talking about occupation has the requirement of a ideology of nationalism. I doubt nationalism existed at that time you are writing about. Anyway most powers were defined by the region of influence which means for example they were able to raise taxes or enforce their laws. Not even the Roman Empire was in nowadays sense a nation. As long as taxes could be raised Rome was satisfied and the Chinese - Vietnamese history will not be very different. From my country Norway I know regions changed loyalty as soon as a new ruler promised better conditions or exchanged the leading aristocracy against his own people.

Exaggerating nowadays understanding of nations to the past is totally wrong.

I think this is an interesting question and one that I've been wondering about for a long time. In that era people identified themselves with the towns and provinces they were born in so I would assume that Vietnamese people who lived at the time of Chinese rule would consider themselves Giao Chi (Jiaozhi) people of Annam (Annan) people, after the name of their province. As to whether they considered themselves Chinese people, I think we should look at how Chinese people saw themselves. Chinese people saw themselves as the Centre of civilization,the Middle Kingdom, surrounded by barbarians. Even during the time when China was split into different kingdoms, all of them still identified themselves as Zhonghua, huaxia people. Im not sure if Vietnamese people felt the same sense of belonging to this civilization? It also gets confusing because Vietnamese emperors did refer to Vietnam as Trung quoc and vietnamese as han to separate Vietnam, a Sinitic civilisation from the neighbouring countries of Indic civilizations

I think this is an interesting question and one that I've been wondering about for a long time. In that era people identified themselves with the towns and provinces they were born in so I would assume that Vietnamese people who lived at the time of Chinese rule would consider themselves Giao Chi (Jiaozhi) people of Annam (Annan) people, after the name of their province. As to whether they considered themselves Chinese people, I think we should look at how Chinese people saw themselves. Chinese people saw themselves as the Centre of civilization,the Middle Kingdom, surrounded by barbarians. Even during the time when China was split into different kingdoms, all of them still identified themselves as Zhonghua, huaxia people. Im not sure if Vietnamese people felt the same sense of belonging to this civilization? It also gets confusing because Vietnamese emperors did refer to Vietnam as Trung quoc and vietnamese as han to separate Vietnam, a Sinitic civilisation from the neighbouring countries of Indic civilizations