The short story "The Red Convertible" by Louise Erdrich is filled with conflicts. There are examples of man vs. self with Henry, especially, as he struggles to find a way to return home and fit in again—this proves impossible because the war has changed him so much.
Henry and Lyman have some conflict between them (man vs. man), but I don't think this is central to the story. Man vs. society is not such a far stretch—the war has taken its toll on Henry and his family has hopes and expectations of him that he is unable to meet.
Looking back over the story, the first part is really enjoyable. These two brothers spend a lot of time together and know each other well. They are able to buy a ostentatious car, pick up girls, travel (all the way to Alaska), and laugh a great deal "before." Everything, of course, changes when Henry returns from Vietnam—a different man: a stranger to his family and even to himself, perhaps.
My opinion is that the central conflict is man vs. self, but I think is it fair to say that it applies to Lyman and Henry. However, in that Lyman is the narrator, and able to tell the tale of Henry's "before and after," perhaps if I had to choose between the brothers and their conflict, I would choose Lyman's, as he struggles to bring his brother back (mentally), witnesses Henry's death, and is left to make sense of it all. (At the same time, it is Henry's internal conflict that causes Lyman's internal conflict...)
The conflict about coming to terms with regard to how Lyman deals with Henry's change and ensuing death are reflected in the lines that tell how their little sister, Bonita, took their picture.
Henry leaned his elbow on the red car's windshield, and he took his other arm and put it over my shoulder, very carefully, as though it was heavy for him to lift and he didn't want to bring the weight down all at once.
"Smile," Bonita said and he did...That picture. I never look at it anymore.
Lyman senses after a while, strangely, that his brother's smile in the picture has changed, but I think it is Lyman who is changing: his struggle continues as he tries to find answers that could help him understand what happened to Henry.