The Soviet Union was brewing a plan to kill three birds with one stone, so why did it assist Vietnam for more than 20 years?

I think there are three main reasons why the Soviet Union assisted Vietnam for more than 20 years:

The first is because the Soviet economy developed in the 1950s and 1960s. Then at this time, the increase in comprehensive national strength gave the Soviet Union the ability to provide foreign assistance. This was the prerequisite material condition for his aid to Vietnam.

The second reason is that after the French army left Vietnam, the United States immediately sent troops to Vietnam. At that time, the competition between the United States and the Soviet Union for hegemony was not over yet. Of course, the Soviet Union could not just watch the United States control Vietnam. Therefore, the Soviet Union decided to aid Vietnam and help Vietnam drive away the American army. This is not only for the independence of the Vietnamese people, but also for achieving a higher status in the competition between the United States and the Soviet Union for hegemony.

The third reason, I think, is related to natural resources. Because most of the Soviet Union's territory is in the northern temperate zone and lacks tropical areas. Although the coastline is very long, it is also subject to ice all year round. Countries like Vietnam in tropical and subtropical areas were very popular for the Soviet Union.

Based on the above reasons, I think the Soviet Union needs to improve its international status and be attracted by Vietnam's geographical location. This is the main reason for aiding Vietnam.