Roaming is ok if you dont really use it! I was with 3 when I was overseas last year and it was free to receive calls, but the surcharge was added to SMS which cost me a bucket load! Try and find out what your plan is!
Vanuatu is one of my fave holidays. We went there for our honeymoon and stayed at Erakor Island. Lovely snorkeling. Yes get reef shoes - they don't really have sand!
People are lovely, food and goods can be pricey and if your bringing anything shell related back don't get anything with seeds on it - I lost some lovely bracelets due to having seed embellishments that I had no idea about!!!