The quake struck at a depth of 164 kilometres (102 miles), 76 kilometres northwest of the town of Isangel on Tanna island, USGS said.
No tsunami warning was issued.
Vanuatu is part of the "Ring of Fire," a zone of tectonic activity around the Pacific frequently subject to earthquakes and volcanic eruptions.
It has been rocked by a series of quakes this year, including some that prompted tsunami warnings.