Most historians do not hold that view. There is a small group that hold fast on this 'stabbed in back' angle to the Vietnam War, meaning that the military could have won the war had not domestic forces undermined it. This is a recurring theme in countries that have lost or struggled in military conflicts. You can find this is works written by veterans like William Sorley (who argues Westmoreland botched the war in 1965-68, but General Abrams was winning the war thereafter) or by a newer generation of historians that set about looking for lost or 'forsaken' victories, like Mark Moyar (who shares Sorely's view but also blames Kennedy for dropping his support of Ngo Dinh Diem when the war was about to be won).

These arguments are in many aspects counterfactuals. Meaning that they read into the future to see what would have happened had X, Y, and Z changed. Those things are not historical -- we don't know the future so we can't write it. The Vietnam era was also a time of incredible information production. Quite literally warehouses of information unused. There were a variety of opinions among officials. These histories ( generally labeled 'revisionist') can sample selectively form this vast array of data and the spectrum of opinions to form an narrative depicting imminent US/RVN victory. However, these same officials could and were often wrong in their opinions. That data was often less 'data-y' than it looked. For example, if a hamlet security evaluation relies on the province chief to assign a letter rating the security of his province, and each year he decides it improves, then you will have data showing that South Vietnam is more secure. Yet it's not empirical data. It's qualitative, subjective assessments/guesses made to look like empirical data. In other words, not reliable.

For an excellent book on the problem of these arguments and the unreliability of data see this book, written by a US Army officer and professor at Westpoint:

Greg Daddis, No Sure Victory (2011).