Well, here is what you have to ask yourself about whether any candidate future war would become an equivalent for the US's Vietnam War, if there was no US Vietnam War. This would apply to the Angola situation, as well as any other.
Why would the US intervene in this other country (that it did not in OTL) with ground forces, including draftees, over a long a period?
If the United States did so would progress/victory seem elusive? Would the cause seem morally controversial? Would there be domestic youth and elite dissent and division over the worthiness of the war?
Why or why not?
The answers to those questions, in the case of, for example, an Angola War (mid-70s), tell you whether that would be a Vietnam equivalent or not.
You can apply the same set of questions to other possibilities like - would an intervention in the Nicaraguan revolution in the 1970s become a Vietnam equivalent? Would an intervention in the Iranian revolution, or hostage crisis, in the 1970s or 1980 become a Vietnam equivalent? Would the intervention in the Lebanese Civil War become a Vietnam equivalent?
If you want to look for examples starting earlier, you could ask if intervention in South Yemen and the Aden Emergency (late 1960s) could have become a Vietnam equivalent, or as you suggested intervention in Ethiopia against the Dergue (1970s) , or intervention against Sukarno's Indonesia, and in favor of Malaysia, in the Konfrontasi (mid-1960s) could have become a Vietnam equivalent.
Personally, I tend to think of US direct involvement in the Indonesian-Malaysian Konfrontasi, or on the Arabian peninsula in the Aden Emergency against the emergence of Communist South Yemen in the 1960s, or intervention in Oman against the Dhofar rebellion in the 1970s would be highly unlikely and unusual, simply because the US tended to see those areas at those times as being in the British (& Commonwealth) sphere or the sphere of local client states like Iran and Saudi Arabia, not traditional areas of US deployment.
Since US intervention is unlikely overall, a Vietnam equivalent wouldn't be possible there. In any case the "western" or anti-left side won there in all cases except South Yemen 1967, and the world yawned and barely noticed the latter.
The Horn of Africa does not seem suitable to me for direct intervention by US forces. The US would be disappointed by the Marxist coup against Haile Selassie in Ethiopia, but surprised by it, and wouldn't have any usable tools to intervene as Ethiopia turns to Cuban and Soviet aid except to try to covertly contact and bribe some junta members and hope power struggles go their way. There's no rationale for up and invading Ethiopia. The US can go ahead and do what it did in OTL and begin to generously fund Somalia for proxy fighting against Ethiopia, if, like OTL, Somalia ditches its former Soviet bloc ties.
Angola - The US will probably at first regard Holden Roberto's FNLA faction, based in the north, which received Zairean support, as the least unacceptable. From the 60s through early 1970s, while the MPLA was getting Cuban and Soviet support Jonas Savimbi's UNITA was getting Chinese and North Korean support. By 1975 it was getting South African support.
A lot of US administrations (not all, very conservative Republican administrations would not mind) would be very wary of a public policy of sending US troops to side with either Portugal in Angola or siding with a faction seen as being allied to South Africa, for domestic political reasons. But foreign policy hawks will be advocates of sending in troops to "arrest" the Cuban troops who are daring to intervene.
If the US government side picks a faction, there is a high chance that the black American community does not embrace it as a representative one and regards that faction as Uncle Toms, so interconnections with internal racial issues still may override ideological/geopolitical motives to intervene directly. But maybe not.
How would a US intervention in Nicaragua go? Well I don't think it could be a Vietnam. Somoza and his family would be a horribly embarrassing ally. The US could be tempted to give them the Diem family treatment, but the Nicaraguan national guard, with full indirect US support, and certainly with US troops, could keep the Sandinistas from taking power over the country, with knock-on effects for other rebel movements in Central America.
If, like in OTL, the US is intervening, 'peacekeeping' in the Lebanese civil war, trying to boost up the Gemayel government, escorting the PLO out, trying to keep Lebanon out of pro-Soviet Syrian clutches, Syrian-Iranian backed terror groups may strike at the US presence.
In the ATL, without the Vietnam war experience, blowing up the US embassy and marine barracks would *not* cause the US leadership to turn tail, the US would be more likely to double-down on peacekeeping and nation-building in Lebanon, with uncertain long-term results.
