It's sounds like being leftist isn't the issue driving US foreign policy, since, as you yourself point out, we do business with Vietnam and China.
Moreover, why would you think that not interfering with foreign states would reduce immigration? To use your Vietnam example, immigrants from Vietnam doubled in the 80s and then doubled again in the 90s. And most undocumented immigrants living in the US today are from Mexico, and I'm not sure how much energy the US has exerted in undermining Mexico's government.