Angola would have some pretty big differences to Vietnam that should mean no intervention long before soldiers arrive. For one, there are no really effective, useful neighbours (only Zambia right?) akin to say China or otherwise, to funnel supplies/support though, for another, Angola was a long way from any potential supporter and required any supporter to be able to get to Angola without interception.
If America was going to get more involved, ala boots on the ground, surely they would first put in place a naval task group that would interdict the air and sea ports from unfriendly nations. The supporter would either need to break that blockade or funnel support through Zambia, who, if supportive, would be incredibly vulnerable to raids and rather an awkward conduit