The nations that have escaped the Soviet Union are mostly our friends and allies now, we all knew the USSR was Russia and mostly used the words interchangeably, and no, we were not trying to conquer and absorb South Vietnam and make it part of our nation.
That’s how you’d like to envision the USSR, but the fact is it was NOT just Russia.
So. Vietnam asked for assistance, and we responded. The ethnic Russians asked Putin for assistance, and he responded. They were being marginalized in areas where they were clear majorities, with the use of Russian alongside Ukrainian in government business (for example), so they decided they didn’t want to be part of Ukraine anymore.
I won’t carry water for Putin or the Russians living in the Ukraine, but look into the causes of this conflict before assuming this was just a bully attacking a neighbor. The Russians within Ukraine’s borders see a different bully.