That vet is mistaken. The NVA and Viet Cong invaded South Vietnam. They weren't "defending thier country", they were invading a soveriegn nation.
The US bombed North Vietnam in order to stop the invasion.
Maybe you can answer a question. Did we have a treaty with South Viet Nam, like SEATO or something? If so, we were honor and duty bound to come to the defense of the south.