I am listening to a podcast on WWI that was discussing the spontaneous Christmas day cease fires and exchanges along the Western Front in the first year of the war. French and German troops exchanged chocolates and cigarettes, and sang Christmas songs together. Hours before this, any hand raised above the trench would have been shot off in moments. Days later, the soldiers got back to the business of slaughtering each other. My understanding of what Vonnegut is saying, is that there is no sense in honoring veterans. Anyone that fights in an armed conflict is a veteran. It is a dumb idea to honor that. My father fought in Vietnam. My impression was that he saw the veteran experience as one that concerned veterans. IMO his emotions and thoughts on the matter were so complex that they could only be diminished and distorted by our decision to honor them. IMO the best thing about honoring veterans, is that people typically shut up and listen to them for a moment. Veterans fill in by choice or not when we are too lazy or too apathetic or too stupid or too scared and people are going to die on our behalf.
I feel similar to you but what I think he was saying is that Armistice Day was a celebration of peace, the thing veterans fight for, after four years of unprecedented slaughter. The moment of silence and the initial clarity or God speaking to humanity is forgotten by making it a blanket holiday for soldiers. He can throw Veterans' Day over his shoulder and Armistice Day remains sacred because essentially we've forgotten why it was so transcendent that the fighting stopped and twenty years later it started again, worse than before. We never learned the lesson. Kurt Vonnegut did but witnessing the firebombing of Dresden probably helped him.