How was the Vietnamese spirit of nationalism developed?

Historical Identity
Vietnam, like most Southeast Asian countries is a natural nation-state. This means that because of a long history of centuries (nearly 1000 years) defending a particular set of borders and establishing a particular national identity, the concept of a "Vietnamese Nation" was more or less universally accepted both by Kinh People (the ethnic majority of Vietnam), by Non-Kinh who had historically been part of Vietnam, and by Non-Kinh who were outside of Vietnam, like the Cambodians, Laotians, and Thais.

What really led to the development of the Vietnamese identity was the constant attempts by China to invade and overrun Vietnam. This created a strong level of patriotism and a unique identity as the "resistance to the Chinese". (Such kinds of resistance-based identity are common and very powerful, such as the Irish Identity, the Kosovar Identity, the Indonesian Identity, and the US Identity.) When compared to the neighboring Laotians and Cambodians, the Vietnamese embrace of more Chinese-style religion (Daoism, Ancestor Worship, and some aspects of Buddhism), made the Vietnamese distinct from the Theravada Buddhism that prevailed among the other two groups.

Modern Vietnam
Modern Vietnamese nationalism and the communism is a direct result of resistance to the modern French Occupation in Indochina. Ho Chi Minh and other Vietnamese saw communism as a form of liberation for the Vietnamese people who had been ravaged by the exceedingly capitalistic removal of their resources for the French industrial machine. The First Indochina War, when Vietnam pushed for independence from the French led to the cultivation of a new Vietnamese resistance-based identity on top of the resistance to China. After this, the war between North Vietnam and the United States, in order to reunify the country, also led to a surge in nationalism and a strengthening of the Vietnamese identity.