The way you phrase the question regarding if their experience in the war "either lead to continued patriotism or disillusionment" makes me think that they somehow volunteered out of patriotism.

There is a very consistent anti-myth amongst the popular reading of the Vietnam War in which we are informed that more men volunteered than were drafted to go to Vietnam. This is very much true but the reason to why so many chose to volunteer were because they saw their draft as something inevitable and believed that by volunteering, they could have one last act of control in choosing their fate. There were of course plenty who volunteered out of patriotism or who came from military families and saw that it was their duty to participate, but there were also those who were unemployed, had family/money problems or simply wanted to get out on an adventure.

Thus, the experiences of volunteers and draftees are in the end very much the same (if they all ended up being 11B infantry men or riflemen of course). Once more, the idea of disillusionment is also a very popular modern myth. The idea that disillusionment would have been widespread before 1969 is not entirely true. Like the above reasons for volunteering, the reasons to why one kept fighting or believe in the US cause were different. It includes actual patriotism, doing your duty for your country but also feeling a hatred for communism or feeling a threat of communism expansion which could lead to the shores of California. There were plenty of men who stopped believing in their cause but many who still to this day believed they did the right thing.