There were no immediate reports of damage or casualties, and no tsunami warning was issued.
Vanuatu is part of the "Ring of Fire", a zone of tectonic activity around the Pacific that is subject to frequent earthquakes and volcanic eruptions.
The quake struck at a depth of 131 kilometres (81 miles), 92 kilometres northwest of the town of Isangel on Tanna island, USGS said.
Vanuatu has been rocked by a series of quakes this year, including some that prompted tsunami warnings.