In short answer, Russia and Cuba are among the most admired countries in my minds.
Why Russia? Why Cuba?
Simply because it was Russia and Cuba who strongly supported the revolutionary movements as well as the national independence of Vietnam, even in the darkest and the most critical periods of modern Vietnamese history, such as during the U.S. escalation of the Vietnam War (1965–1968) or the period of 1978–1979, when Vietnam was totally surrounded in all directions by hostile forces led by China.
“Мы солидарны с тобой, Вьетнам!” (“We stand with you, Vietnam!”) - a Russian poster about the Vietnam War in 1970.
Also, because it was Russia or the former Soviet Union who successfully defeated the Nazi Germany and its genocidal regime in Europe, paving the way to the decolonization and national liberation movements worldwide in 1950s–1960s.
Later on, the Russian-made AK-47s, T-54 tanks and S-75 missiles successfully safeguarded the national independence of a lot of countries in Africa and Asia, such as China, Mongolia, India, Indonesia and Vietnam during the 1960s-1970s. Moreover, the Russian educational system successfully trained the first generation of Vietnamese engineers and technical experts.
Cuban President Fidel Castro visited the liberated zones of Quảng Trị Province, Central Vietnam in 1973.
Also, because it was Cuba who provided the enormous diplomatic and medical supports for Vietnamese people, amid the aerial bombings of the 1960s or the economic sanctions during the 1970s. Remember at that time Cuba itself was also totally surrounded by hostile forces. In case of Vietnam, at the high peak of the Vietnam War, Cuban President Fidel Castro once said:
“For Viet Nam, we are ready to sacrifice even our blood”
Later on, it was the Cuban volunteers who fought decisively against the Western mercenaries and troops from the genocidal regime of Apartheid in South Africa, successfully protected the independence of the Republic of Angola during the bloody civil war and the foreign intervention (1975–1988), also safeguarded the birth of Namibia as a newly independent country from the Apartheid regime.
Nowadays, Vietnam has always maintained very good relationships with these two countries. In terms of diplomatic language, the Russia-Vietnam relations is the “traditional friendship”, while the Cuba-Vietnam relations has been regarded as the “special relationship”.