Rotation of Forces
I believe that the UK is currently operating a six-month rotation in Afghanistan with each deployed member granted two week's leave in the UK during that period. Whole units, or "Battle Groups" (mixtures of Infantry, Engineers, Support etc) are deployed so that would seem to be the "unit" rotation in operation. Beat-up training pre-deployment is intense, with the troops assigned to deploy, working and training together.
However, Afghanistan is some 40-50 years after the Vietnam War and a lot has been learnt since then and much put into practice. It could be argued that the UK Ground Forces (not just Army, but Royal Marines and elements of the RAF Regiment) had the best training in the World for (admittedly lower intensity) "COIN" Operations with Op BANNER in Northern Ireland from 1969 to, arguably, the present day (Army fatalities were incurred in 2009) plus previous deployments in Malaya, Aden, Cyprus and as far back as Palestine and Greece at the end of WWII and so have approached “yet another COIN Op” with a clear mindset; Wars at the “Twilight of Empire” have been their forte since, probably, Suez in 1956.
The benefit here could in some way be down to the fact that since c. 1960(ish) HM Armed Forces have been volunteer only, following the end of National Service (i.e. "The Draft") post WWII and "Institutional Memory" is strong with many soldiers serving 22 years "with the Colours" (longer if you look at "Late Entry" Commissions of WOs to Officers), so lessons and experiences are passed on from one generation to the next. Add to this the fact that HM Armed Forces, whilst not being as lavishly equipped as some forces are without doubt the best trained and amongst the best led on the planet and you can get an idea of how the system works. However, that is “now” and the general thrust of the initial post was how the US could have bettered its performance in Vietnam some 40-odd years ago.
As pervious posters have alluded to, the mind set of politicians and senior military commanders must be taken into consideration. Many “Seniors” (probably from the rank of Lt-Colonel upwards) were WWII veterans and their direct experience coupled with the prevailing Strategic thinking of the time would have influenced how they behaved and indeed viewed the conflict as a whole.
With the exception of the Philippines and some small scales interventions in Latin America, recent (i.e. prior C20) US experience was WWI, WWII and Korea; all three were “classic” wars of army vs. army, ship vs. ship and, latterly air to air combat. Semantics apart, America never had an Empire. The “West” was expanded and settled with a vastly inferior (technology-wise) indigenous population to oppose it. The Spanish-American War of 1898 was a “War of Liberation” with the US (outwardly) fighting FOR the Cubans and Pilipinos AGAINST the Spanish Imperialists and the forays into Latin America were short in duration and limited in aim. In short America, bereft of a “Classic” Empire in the style of Britain or France’s, never had to interact with populations who were, in general, nothing short of ecstatic to see them as they were there to throw out Spaniards / Imperial Germans / Nazi Germans / Communist Koreans…
Vietnam was a shock in the truest sense of the word. Add to the fact that Ho Chi Minh was George Washington (OK; but you see what I mean), that some of the Vietnamese people seemed to openly support him and couple that with flawed military and political policy and the end result was never going to be in doubt.
Full Unit rotation, intense training, calling up reserves and (IMHO) not sending draftees may have helped, but without a change in mind-set, I do not think that it would have been enough. For the reasons I have attempted to set out above, those in the higher levels of command saw nothing wrong with how they were prosecuting the war. How could they, based on all that they had (personally) seen and “knew”?
The fruit of this bitter learning process is only now coming to the fore in Afghanistan with no-one (?) in the US Forces with first hand recollection of Vietnam and a string of recent successes to draw on (Grenada ’83, Panama ’89 GWI ’91 and to a lesser extent Peacekeeping in the Balkans).
The US has always been a “War Fighting” force par excellence, but it has been argued this is based on its ability to fight “Industrialized War” (i.e. WW2.5) and that large scale COIN is “not their thing”. In short they are finally adapting to it, but it could not have happened back in the 60s and certainly not “overnight” once embroiled – after all; Armies fight the last war, don’t they?