The issue was not the technical quality of the US forces. The major difference would be in the air war over N. Vietnam where smart bombs would take down bridges and other installations quickly and with minimal losses, also modern US a/c & weapons would be able to take down air defenses fairly quickly. Saying that, if the US had the same rules about targets etc, the effect would be relatively small.
On the ground, in the south, the problem was always that the South Vietnamese government never did what was needed to offer the people something better than what the VC/NVN offered, at least as the locals saw it. Absent a strategy of land reform, providing services with much less corruption etc, the war was not winnable by the US.