I believe I did answer your question. The difference between Vietnam and Eastern Europe was that in Eastern Europe, the Russian military was already in there. For the United States to keep going toward Russia, we and the Western Allies would have had to face well-entrenched Russian and Soviet troops who were all set up in Eastern Europe already, just waiting for us to arrive. That is a far more difficult task than going to a country and fighting a native foe who might be supplied with arms by Russia, but not actual Russian manpower in there.
I don't personally believe that going into Vietnam was a very good idea, we should have not have done it. The people of Vietnam were getting ready to elect the leader who threw the colonial French out, Ho Chi Minh, and while the US didn't like him the fact is that it was the Vietnamese' country and their right to elect any leader they wanted. But to answer you question, the reason we went into Vietnam was that with the Soviet Union's brutal domination of Eastern Europe, the US felt it was compelled to stop the spread of Communism-even if the Vietnamese wanted to freely elect a Communist leader.