I do not believe equating Vietnam with Ukraine is something you can do in good faith.
Edit: America has remained an empire through a willingness to meddle far from home unde the banner of "protecting American interests". Watching a so-called conservative president dismantle the American empire is startling, seeing the rank-and-file fall in line with narry a dissention is almost unnerving. Anti-imperialist Americans must be having mixed and conflicting feelings right now.
1. The whole Korean peninsula was a colony of Japan, and was divided between the US and Russia after WWII. Both sets of governments claimed to be the the legitimate authority over the entire region. It ended up being a proxy war - so not comparable to Ukraine for a different reason, unless one thinks the war in Ukraine is a proxy war - which completely takes away the agency and sovereignty of the people of Ukraine, and will be provable via a natural experiment over the next days as the US has stopped its involvement, the the war is continuing.
And your same logic would have gotten the US involved in the korean and vietnam wars as well. How do you distinguish those?