I also don't think there are any liveaboards operating in Vanuatu.
As for diving there, the number one spot has to be the Coolidge which is a wreck the size of the Titanic done as a beach dive from Luganville on Espiritu Santo. I dived with Aquamarine and certainly recommend them. For accomodation I stayed at Deco Stop Lodge and they were brilliant. It is set up for divers and has wash tanks and a drying room. Aquamarine pick up from there and usually go straight out to the beach where the Coolidge dive start and after the dive straight back to Deco Stop - all really nice and easy. Food at Deco Stop is really good and Kim and Ben who run it are as helpful as can be.
There are reef dives around Luganville too. I dived Tabuta Point and Chails Reef and really enjoyed the dives. There is also a dive at Million Dollar Point where the US military dumped all their jeeps, trucks etc at the end of the 2nd World War.
On Efate, the main Island, I have stayed at the capital, Port Vila, and dived with Big Blue. There are some nice wreck dives and reasonable reef dives, the Cathedral is a great dive but very dependent on conditions.
From Port Vila you can dive at Hideaway Island as a day trip which was good value. There is some good diving there and some superb snorkelling too.
That's the only places I have dived in Vanuatu so far but I'll be going back!!
By the way, Espiritu Santo is an hour or so plane journey fron Efate, as is Tanna so if you want to island hop factor in internal flights.