The only reason the Korean War was undertaken by the UN and not Vietnam is because during Korea, the USSR was boycotting the UN for recognizing Taiwan as the true China immediately after the communist government took over and didn't have veto power in the Security Council. They were an active member of the security council during Vietnam and would have vetoed any resolution that didn't support the Viet Minh.

If that's literally your only reason, I think you might want to look back and re-examine that "international consensus"