The first part is the boot camp part where Private Pyle eventually loses his mind and kills the drill sergeant. Then, like a snap of the finger, we are with Joker in Vietnam and the second half seems to be completely disconnected, save for the characters of Joker and Cowboy. Nothing really is mentioned in the second half about their experience in the first half, and it just seems generally disconnected. Why did Kubrick set the movie up like this, and is he on record explaining that?
It can be argued that Joker is the main character since he narrates the film and it basically follows his experiences. During the time period depicted (1967-68) soldiers were commonly sent directly from boot camp to assignments within Vietnam. In the film, all of the characters who were training at the bootcamp get their MOS assignments (Joker gets Military Journalism, all the rest get Infantry) at their graduation. That night is when Pyle looses it and shoots kills Gunnery Sergeant Hartman. From there the film jumps directly to Vietnam which is where Joker was stationed. It just happened that in carrying out his job (covering the war for Military publications) he came across a unit that contained soldiers that he had gone to basic with. If you view it as Joker's story, it's not really disconnected at all - it just has a major setting change at the midpoint.
I don’t think Joker completely buys into the military mind-set the way the others do, so it not very likely he will talk about boot-camp with other members of his graduating class when he runs into them.
I don’t know what basic training is like now, but when I went through it (early 70’s) it was no high school that one might sit around reminiscing about for years after. In basic, you were a “raisin” or a “maggot”, and the military goal was to break down any sense of individuality you might feel and remold you as part of a unit-to think alike, move alike, and to fight alike. Not everyone survives. (Three recruits committed suicide in my class and 5 went UA). The pressure was intense to stop thinking like a boy, and to leave behind civilian ways. Graduation was a symbol of that transition, so after there was a feeling of "that was then, this is now".
Joker manages to remain aloof from this in his own way, to see things from an objective viewpoint as a budding journalist. Chameleon-like, he can mouth the jargon, but still understand the dichotomy between the need for peace and the desire to kill. (re his discussion about the peace symbol).
And then there was the need to separate the reality of the everyday boredom and butchery from the “World” back home: daydreaming about the “world” can get you killed in the field. Every thought was geared towards military life, and when you had time to yourself you were “coking and smoking” (not what you think) or getting drunk and high.
People nowadays are more open about expressing their feelings, but back then it was considered weak. Maybe if more returning vets had been able to talk about their experiences they wouldn’t have come back so FUBAR.