500N,
I shall defer to Hollywood on this one....the only times I heard a call for help go out about units being over run it was done in plain English.
When you hear that kind of talk it is heart stopping in effect.
Then.....it is like a "Hey Rube!" call at the circus.....everyone that can help heads that way.
Generally it was Platoons that got over run although some Fire Bases had that happen. In some situations units called friendly artillery down on their own positions and in one instance called Napalm on themselves. A 101st Airborne Company Commander named Carpenter did that and got the DSC for doing so. Oddly, none of his guys were hurt and the Napalm broke the Enemy attack. Carpenter played the "Lonesome End" on the West Point Football Team.
The classic case of a unit being over run was the Special Forces unit at the Lang Vei SF Camp not far from Khe Sanh. The NVA attacked using PT-76 Tanks. The only weapon the SF guys had to counter the Tanks were LAW Rockets that proved damn near useless. The QRF for the SF guys was the Marines at Khe Sanh.....who stayed home.....but to be fair it was more high levels of command that caused that rather than the Marines at KS itself.
Feb 6-7, 1968: Battle of Lang Vei Special Forces Camp | Vietnam War | Command Posts