First of all, the typical asymmetrical warfare happens between two camps where the weaker side are backed by substantially by other forces. Like Vietnam war and Soviet-Afghan war, for example, Vietnam were backed by the Russian and Chinese, these countries supply weapons and other necessary logistics to support Vietnam. In Soviet-Afghan war, all Muslim countries and the US plus Pakistan supported Afghan resistance. It is very different from the Chinese civil war where the remain forces get no backup.

China has never went into a war far away from its land, it lacks of the ability to project force to non-adjacent area. After all, it can't even "liberate" Taiwan, so don't even talk about force projection.

Other factors such as the resolute to put off resistance, a country has to weight its options and cost when dealing with war. Brute force may not be the best options. Especially, for example Vietnam. The American's objective was to stop Communists, Russian in this case, to expend to the South East Asia.

However there were other options, such as making contact with China, since China wasn't in a good term with Russian. Later, we certainly see that Vietnam was contained.

Don't to be so narrow minded.