Let me repeat: no one is advocating US ground forces. There was a period in 1973-4 for ARVN to revamp their strategy, and they didn't take advantage of it. There was no recon of NVA troop buildups. Why? Because their F-5s (useless against Fishbeds) without ECM equipment would be blown out of the sky by NVA SAMs. Personally, I see three Vietnamese problems in 1973-5.
1) ARVN officer corps: the NCOs fought bravely to the last man. Many of their officers deserted their units (Thieu the foremost, loading his Taipei- bound 707 with as much looted gold as possible), and were selected (esp O-4 to O-7) for their political connections rather than military skills. Many generals were average or better, but the battalion/divisional level was often dead wood.
2) Thieu's interference: "Don't surrender an inch of territory" might sound nice on paper, but they did not have the supplies. If you can only fire 4 artillery rounds a day due to supply problems, don't try to defend 600 miles of military insignificant borderlands.
3) Lack of proper equipment: I've outlined that earlier.
Perhaps one of our resident military experts (which I am definitely not) can outline a plausible scenario for a RVN victory in '75?