Looking at a map printed in an Asian ethnomusicology book from 1939Kunst, Jaap, 1939, Music in Nias. Leiden, Netherlands., I read about Nias, a small island in the Indian Ocean off the western coast of Sumatra and part of Indonesia. Though not close to Vietnam, the Nias and Vietnamese share one similar instrument: the druri dana (Nias) and the đao đao (Vietnam).
On one of my many trips to the National Library of Singapore, I stumbled across this old book...