There are a number of key differences in the nature of the combatants and the background to the struggles themselves that have necessitated a different approach in the case of the Taliban in Afghanistan, from both sides in the conflict.
First, let's note that the struggle in Afghanistan is largely religious in origin and motivation. While it is still anti-imperial (or one could make that argument) just as it was in Vietnam, religion and the differences between radical Islam and the West are primary drivers in the conflict. Nationalism and communism were the main motives in the South Vietnamese insurgency.
Second, the Taliban have no foreign sponsors that are as well-armed or generous as the Soviets and China were to North Vietnam and the Vietcong. The Taliban have virtually no anti-aircraft weapons, and certainly not sophisticated SAMs and large caliber guns.
Thirdly, I would argue the Vietcong were better trained (overall) than the Taliban, who try to make up for in zeal what they lack in fighting skill. I am not trying to be dismissive, as the Taliban have mounted some fairly impressive, coordinated assaults in recent months, but from all accounts I have read, their aim is abysmal, and they pose more of a threat by their continued existence than they do on the battlefield. The Vietcong, by most standards, represented a more dangerous, skilled fighting force.