hi,
vanuatu is a fav of ours - every 2 years we go back there. you fly into Port Vila and connect from there, depending on where you're going - depends what you want to dive and see.
if its the Coolidge, then you need to fly to Santo - when there we prefer staying at the Deco Stop -
http://www.decostop.com.vu/ -and diving with Aquamarine -
http://www.aquamarinesanto.com/ - or for a bit extra, check out Bokissa Island -
http://www.bokissa.vu/
we always visit Hideaway Island -
http://www.hideaway.com.vu/ - , about 15 mins by bus and a small lagoon boat ride from port villa. heaps of dive sites from 1 min to 15 mins by boat, good prices, accomodation is from dorm to good clean bungalows (no tv, radios, air conditioning) excellent dive staff and great restaurant on the beach. easy diving, good dive boats, various reefs, they have 2 smallish wrecks also.
also check out Tranquility Island, about an hour by bus and boat from port vila -
http://www.southpacdivecruise.com.vu/dive.html - very very basic (eco) but worth it. its a turtle santuary also so you can adopt a turtle if you want. you can do this as a day trip inc lunch and transport. we thinking of staying here for a few days next time we visit. Thier prices by the way inc meals - quite remote so you cant head out to the shops.
when we go, we always go between may and july but be aware we have school holidays approx early july and Hideaway has a photography festival late may. may to july is our winter here in the southern hemisphere and i personally wouldnt want to go in jan because of the humidity, but thats a personal thing.
i hope you enjoy your time in vanuatu!