Tanna felt like you were really getting away from it. The island is about 45 min in a plane south of port vila and the climate is sub-tropical, with lower humidity than vila. There are only dirt roads and no electricity and there is only a few small low key resorts which have generators. Apparently most of the island grows organic produce, which makes for fairly simple but great tasting food. Here is where you have the best chance of seeing the locals living traditional lifestyles, even those who worship a cargo cult - a truly weird & wonderful concept.
I stayed on the west coast of the island, at white grass resort. This resort was low key, basic and really friendly, with great local music some nights.
The yasur volcano was really spectacular - regular dramatic lava sprays and smoke. A tour takes a few hours of hard driving from the west coast over a very rough 4 wheel drive road, but well worth the trip. The crater is an easy 20 min walk from the carpark.
The snorkelling of the beach in front of the resort was some of the best i have ever encountered, crystal clear 30 metre visability. Great small coral reefs with mostly small species and some sea turtles.
Try one of the local cultural tours - they were really enjoyable.