US also wanted to retake lost territory of South Vietnam.Let me break it down to you simply:
The Vietnam War would be equivalent to Russia sending 500K troops to defend DPR/LNR states-only, and refraining from invading Ukrainian-controlled territory, just as US send 500K troops to defend South Vietnam-only, and refraining from invading North Vietnam territory.
- Russia invaded entire Ukraine with a land force + aerial campaign.
- US and South Vietnam did not invade North Vietnam with a land force, only aerial bombardment.
- ....
- Russia invaded Ukraine despite risk of NATO intervention
- US refused to invade North Vietnam because of risk of Chinese intervention.
- ....
- Not comparable
DPR/LPR covered all of Donbass at the start of Euromaidan, just that they lost it to a Ukrainian counterattack in 2015. So there's some equivalence here.