How can we assess the impact that deploying individually rather than by unit had on those who served those turns in the Vietnam War?
As your question implies, after U. S. Army divisions--the 101st Airborne, the 4th Infantry Division, for example--deployed intact in the mid-1960s, troops came in country and were assigned to units as needed. This was not significantly different from what occurred in the later years of WWII, but the process had serious drawbacks as the Vietnam War continued.
One of the primary strengths of any tactical unit is the result of "unit cohesion," which is created by a group of soldiers or Marines who train together and go to war together. By the time they get into battle, they know each other's strengths and weaknesses and, in the best units, they learn to compensate for those strengths and weaknesses, thereby increasing the chances of survival for the group. As a war goes on, replacements come into the group, but because the group (or what's left of it) already has a sort of integrity, the replacements are usually viewed with a bit of suspicion until they either prove themselves worthy or worthless, and these replacements are often left to shift for themselves--essentially, they either sink or swim on their own. If they act in accord with the existing group's expectations, they are admitted into the group. And then they, in turn, look upon replacements with some skepticism and wait to see how replacements conduct themselves before even bothering to learn their names.
In Vietnam, since most divisions were in place by the mid-1960s, as troop strength built up in 1967 and 1968 especially, almost everyone who arrived came as an individual, often to replace casualties in units that had been in country for one or two years. By that point, unit cohesion existed, but its scale was vastly reduced from the kind of unit cohesion that existed when entire divisions arrived in Vietnam at the same time. And as unit cohesion degraded, performance--in a real sense, survivability--degraded as well. Replacements often felt like bastard children until an existing group accepted them into its circle.
The bottom line here is that, as unit cohesion deteriorates (for whatever reason), performance tends to deteriorate, and even though most troops in Vietnam performed well, the overall performance was negatively affected by the individual replacement process, which was, unfortunately, unavoidable.
It would be exceedingly difficult to assess this in any objective and scientific way. This is true for at least two reasons. First, the war is at least 40 years in the past. Memories have changed or faded. This would make it hard to conduct any sorts of surveys about this issue. Second, even if the war were recent, it would be very hard to conduct legitimate surveys on this subject. We could ask people who deployed individually how they felt. But we would have a hard time determining how they would have felt if they had deployed by unit. We would have to try to find people who were as alike as possible but for the circumstances of their deployment. This would be very hard to do.
Therefore, we are left to make do with anecdotal evidence. We have to look at things like oral histories and whatever documents we can find that discuss this issue. We have to create our own conjectures about what impacts this should have had. It makes sense in our minds that this would cause less unit cohesion and lower morale. Since the anecdotal evidence tends to bear this out, we can assess tentatively that the soldiers who deployed individually felt more isolated and alone. However, this is not a scientific conclusion.