Not open up another debate on Vietnam at this time, but the domino theory wasn't unfounded. For different reasons Laos and Cambodia fell under communist rule, and Thailand faced a communist insurgency that was defeated. Our design for containing communism was arguably wrong. Additionally, China was the smaller player, the USSR provided the bulk of support to North Vietnam.
I agree the domino theory was well founded, but equally I believe the effect of the expansion of "communist" influence in that region was grossly exaggerated. Seriously, with your UW hat on, what other ideology could more effectively rally the energy of peasant farmers tired of being forced to a life of working in poverty the land of some distant, entitled land owner?? The nature of Ideology does not create the problem, in large part the nature of the problem picks the ideology. We would do well to remember this as we continue to sweat over employment of an Islamic based ideology to motivate Sunni Muslims to rally to coerce change of the local and distant systems of governance controlling their lives.

I suspect the Vietnamese were far more concerned about China's designs on their newly liberated territory than we were. Working with the Russians was simply smart business on their part. After all, we weren't helping them, so who else to turn to both force the US to stop meddling in Vietnamese sovereignty AND to protect that same sovereignty from Chinese expansion??

when did capacity building show up doctrinally?
Not sure when it showed up, but it did gain a great deal of traction around 2004/5. We had the success of the "Basilan Model" from Dave Maxwell's great efforts there working with the Philippine military being held out as a better way to apply US support to these types of situations in sharp contrast to what was happening at that time in Iraq and Afghanistan. The Iraq insurgency was in full swing, and the US backed Constitution forming a Northern Alliance monopoly with the associated election of Karzai to lead it was moving the ousted Taliban to revolutionary action to challenge that intolerable condition in Afghanistan. (our actions to counter that revolution in Afghanistan quickly began to grow the parallel resistance insurgency that has occupied much of our energy there since).

I recall specifically "capacity building" becoming a buzz term, and us having to do a bit of research to figure out what exactly the difference was between "capacity" and "capability."