In Walter Dean Myers' Fallen Angels, Richie Perry, Peewee, Jenkins, and others are in Vietnam because they were drafted (or enlisted under draft pressure) into the U.S. Army during the Vietnam War.
The source mentions "Lobel"—the character is Talon (or "T"), not Lobel. Lobel does not appear in the novel. The core squad: Richie, Peewee, Jenkins, Johnson, Brunner, Monaco, Talon.
Their presence in Vietnam isn't accidental—it's the engine driving the novel's exploration of race, class, and the human cost of a war fought largely by the poor and marginalized.