Stones are hard, cold, and tough to break. In line 2, the speaker asks if the people of Vietnam used stone lanterns to light their way. This is a particularly archaic method of lighting, and might indicate that the speaker believes that these people lived in poverty, or were just old-fashioned. But, like we said, stones are hard to destroy; these lanterns would have been long-lasting, like a legacy.
So, is the speaker actually asking about the legacy of the Vietnamese people? In line 10, the second speaker answers that the people's "hearts turned to stone." This is a more common metaphor for becoming joyless and sorrowful (hence the phrase "stone cold"). If the people of Vietnam did have any lasting legacy, the second speaker says, it was made cold and joyless by the war. The stones themselves would no longer be usable as lanterns; they'd be physically cold to the touch, too, just like the hearts of those lost to the war and those left behind in sorrow.