To understand the reason behind nationalism of Vietnamese people, you need to look into our history and and our culture. And then you need to put yourself into the shoes of common Vietnamese people, who work and live every day without thinking much about anything else but what happened to them.
Firstly, we Vietnamese are really close to our relatives, especially our elders. When you visit a Vietnamese person’s home, you definitely find that regardless of his/her religion, political belief, social status… there will always be an altar to pray for that family’s ancestors. Each family has their own stories that has been passed down, outside of textbooks and what the historians agreed up on. When Catholic first introduced in Vietnam, it was a big mistake that the priests requested their followers to get rid of these altars, as “you are all children of God, and your parents are just bodies/vessels that God used to deliver you, his children, into this world” (words of a priest that I met). To Vietnamese, you get rid of the altars means you forget your ancestors, you forget who you are and where you are from, you are no longer Vietnamese. Of course it changed few decades ago, Christian in Vietnam are allowed to have an altar to pray to ancestors at home, next to the place they pray to Jesus, same thing that helped Buddhism took root in Vietnamese more than 1000 years ago. The point here is not about religion, but I am trying to demonstrate how strong the family heritate and relative relationships in our culture. Which will help explain the common people’s thinking.
In the last 2 centuries, we have been almost constantly at war with foreign powers, much more powerful ones, and dominated by them. The French colonial time was harsh and terrible. You can hear nice stories here and there from the upper class but personal experiences of most people were not so great. It can compare to what happens in Middle East with ISIS nowadays. I once told my classmates in US about the story of my grandmother, at the age of 12, and her brothers, sisters, parents watched a French officer cut off her brother’s head in their front yard while making them watched, then his head was put on a stick to display in public. My great grandparents had to steal their son’s head back in middle of the night so they could burry him with a full body. And that was not uncommon thing happened during their time.
Then came the Japanese, during their short occupation, 2 million people out of 20 million of Vietnam’s entire population died of hunger (some might argue about the numbers but it definitely was a lot). Let’s assume the number is quite accurate, then to be more specific, it all focused on the North of Vietnam, which has the population of about 12 million at the time. Now try to imagine being a Vietnamese that lived through that, like my grandmother. 1 out of 6 people you know died from hunger. Entire families are erased, even villages. And it all happened because of foreign powers play war on our soil: French collected food for their own use, Japanese needed to supply their troops in South East Asia, Allies bombed the roads and blocked the ports to prevent Japanese transporting foods from the South to their troops in the North. And Vietnamese died, lots and lots of us: family members, relatives, friends, people you know or heard about….
Later during the war with US, it was not so much different. Constantly living in fear of bombing, jets flying over our heads… and fear of a foreign powerful army that might turn out just to be the same as the last ones (murder the ones that oppose them and starve the ones who don’t). We can all agree that US was not the same as French or Japanese in this case, but to people like my grandmother at that point, it was an assumption: “they might be nice”. But it also means “they are more powerful than the French and Japanese, if they are bad then we are truly fucked”.
That kind of thinking and attitude based on life experience built up over time which became fighting spirit “we must fight for our own survival”, eventually leads to “with our mere will, we won against the world’s most powerful superpowers” (I know it sounds like a propaganda but it makes sense to most normal people), finalizing as “we are Vietnamese and we are no lesser than any other nations in this world”. Now, that sounds proudly and have strong backing facts to it, at least in normal people’s mind. These things were built up by the previous generations and heritated by younger generations, which is the foundation of Vietnamese people’s nationalism. Please remember we listen to our elders first before anything else.
I think the best case to relate to is Jewish people and the birth of Israel. Just look at their history and you can find similarities. There are less Jews in the world compared to 1939 but now they are more proudly saying they are Jewish.