I would recommand with no hesitation the Deco Stop Lodge which is held by an australian couple, Ben and Kim.
They've have perfectly designed and arranged the hotel for divers, with rinsing tanks right behind the rooms taht you can access from your own bathroom. The location overviewing the channel is great, there's a good swimming pool on a wooden deck, with typical nivanuatu decoration and statues.
It's 5 minutes walk from the main street and car rental (Laho). Rent a car and don't miss going to Champagne beach, one the most beautiful beaches I have ever seen, it's a 90 minutes 4WD ride though, and also to Blue hole on the way (in front of Oyster bay).
i've dived with Allan Powers the original dive center in Santo, they're all very experimented on the wreck and they'll pick you up at Deco Stop (like any other dive center I believe).
Have a try at the market for dinner, excellent Santo beef cooked for a ridiculous price. Also have a kava shell at the Kava bar next to the Shell station (10 minutes walk from Deco Stop).
In Vila, Nautilus is the most experienced dive center, I liked to stay at French owned Fatumaru Lodge excellent croissants heated on the barbecue(!) for breakfast. Close to downtown and to the best steak in town at Flaming Bull.
