The difference is that Vietnam was fighting against colonialism and to establish socialism, while Ukraine is being harvested for resources by the west and is used as a battering ram against geopolitical adversaries of the west, including by arming and empowering neo-Nazis against those in the Donbass region. This is just metaphysics, thinking you can take a very specific set of circumstances out of its necessary context and dogmatically apply it to every circumstance.