The IR technology that "first appeared" during the Vietnam War was an attempt to use IR as guidance for weapons--glideable bombs and the like. It would come to fruition much later, in the Gulf Wars, but Vietnam saw the first baby steps.
One of the types that carried this primitive stuff was the Schweizer sailplane-based Quietplanes that flew near-silently--actually you could hear them but they sounded more like a large flock of birds than an airplane--at night, but it was never really employed for its design purpose.