I found this article
It seems that Vanuatu is not doing a good job of protecting their shiny, new world heritage site.

It seems that the Fels Cave on Lelepa Island, part of Chief Roi Mata's Domain WHS, collapsed during the massive earthquake that struck Vanuatu on Dec 17. This is mentioned in a local Facebook post, but not yet verified by mainstream media.

(thanks Shandos for alerting!)

We have re-booked our trip to Vanuatu in June (this time staying at new villas within the buffer zone!) and will be able to provide better info after then. At this point, I have received this clarification from our accommodation about the WHS:

"We can certainly arrange a cultural tour for you to Chief Roimata's domain. Unfortunately the cave is no longer accessible due to the landslide, however the other points of interest in the domain are safe and provide a lot of cultural interest. The local villagers are also in the planning stages of something new near the ruins the of the cave."