Possible response:

Purely Socialist Economy

A purely socialist economy is characterized by public ownership of the means of production, central planning of the economy, and distribution of goods and services based on social needs and not market forces. In practice, no country has ever achieved a purely socialist economy, as various factors such as political, historical, and cultural contexts have influenced their economic systems. However, some countries have claimed to be socialist or communist and have implemented policies and institutions that reflect socialist ideals.

None of These Countries

The countries listed in the question, namely USSR (which dissolved in 1991), China, and Vietnam, have all undergone significant economic reforms in the past decades that have reduced the role of socialism in their economies and increased the role of market mechanisms. While these countries still have some state-owned enterprises and some redistributive policies, they also have private ownership, foreign investment, and trade with other countries. Therefore, none of these countries can be considered as having a purely socialist economy.

USSR

The USSR was a federal socialist state that existed from 1922 to 1991, comprising 15 republics and a centrally planned economy. The government owned and controlled most of the industries, agriculture, and services, and set targets for production and prices. However, the Soviet economy faced many problems such as inefficiency, corruption, shortages, and stagnation. The collapse of the Soviet Union led to the transition to a market economy, with varying degrees of success and challenges.

China

China has been ruled by the Communist Party since 1949 and has undergone significant economic reforms since the late 1970s. The government has allowed private ownership, foreign investment, and market-oriented policies while maintaining state control over key sectors such as banking, energy, and telecommunications. China has become the world's second-largest economy, but also faces issues such as income inequality, debt, pollution, and political repression.

Vietnam

Vietnam has been ruled by the Communist Party since 1975 and has implemented economic reforms since the mid-1980s, known as Doi Moi or renovation. The government has encouraged private enterprise, foreign trade and investment, and market-oriented policies while retaining state ownership and control over strategic sectors such as banking, transportation, and defense. Vietnam has achieved rapid economic growth and poverty reduction, but also faces challenges such as corruption, environmental degradation, and human rights abuses.

Conclusion

In conclusion, none of the countries listed in the question can be considered as having a purely socialist economy, as they have all adopted various forms of market mechanisms, private ownership, and integration with the global economy. However, they also differ in their political systems, cultural values, and economic performance, and have faced different challenges and opportunities in their paths of development. Understanding the complexities and nuances of these countries' economies requires a multidisciplinary approach that takes into account historical, political, cultural, and economic factors.