In an interview with a major (track 8, day 2 in the audiobook), the interviewee mentions having friends who were in Ethiopia, Angola, and Vietnam. Is this likely referring to the Vietnam War (except, of course, supporting the Communists)?
In the context of Soviet military interventions around the world, all three of Ethiopia, Angola, and Vietnam would make sense to be referenced.
The Ogaden War was fought between Ethiopia and Somalia from 1977 to 1978, with Ethiopia being backed by Cuba and the USSR. Thousands of Cuban soldiers fought in the war, and the USSR sent 1,500 military advisors as well as artillery and military vehicles.
The USSR also remained involved in Ethiopia even after the end of the war, from 1978 to 1985 and beyond.
The Angolan Civil War, which lasted from 1975 (following Angolan independence) until 2002, was used as a proxy "hot" part of the Cold War, with one side (MPLA) backed by Cuba and more indirectly the USSR, and the other side (UNITA) backed by South Africa and more indirectly the USA. Soviet troops were definitely involved in Angola.
Even before that, the USSR was deeply involved in Angola, assisting the fight for Angolan independence from Portugal. See for example Christopher Stevens, "The Soviet Union and Angola", African Affairs 75(299) (April 1976), pp. 137-151, an article which opens by stating that "Never before has the USSR assisted an African liberation movement on such a grand scale."
The Vietnam War is generally better known, at least in the West, than either of the above mentioned wars in Africa, due to more direct involvement by the USA, which sent hundreds of thousands of troops to Vietnam in an intervention which is still talked about in US politics to this day.
The USSR's direct involvement in the Vietnam War was smaller than the USA's, with far fewer troops (around 3,000) stationed in Vietnam, although again they also sent significant support to North Vietnam in terms of arms and materials, as well as training Vietnamese military personnel.
So yes, Soviet military officers might have been found in all three of Ethiopia, Angola, and Vietnam - and yes, the involvement in Vietnam would be referring to what's known in the West as the Vietnam War, in which the USSR was also deeply involved.
Not necessarily the war.
The USSR was involved in many conflicts (see Rand's answer), and in most cases retained presence afterwards, whenever it was possible. Having permanent military bases, like in Vietnam, was seen as ideal, but thousands of "military advisors" went through many countries, both during and after the known conflicts. Particularly in the Middle East (Egypt, Syria, Lebanon, Iraq, etc.) In the later years of the USSR Air Force, it was hard to find a regiment where somebody wouldn't go through such "international assignment" in the past. I even had a relative who trained pilots in Angola.
So, "having friends who were in [one of such countries]" doesn't necessarily mean they were involved in the respective wars. Depending on the year of the interview, it could be rather more likely they went through regular military service in these countries. This particularly applies to Vietnam, which had permanent bases until 2000s and in which direct military involvement was comparatively small.