I've heard Royal Tonga Airlines is 'extremely' sporadic and unreliable these days (even less than it used to be), so plan accordingly.. BTW, the flights within Tonga go from Nuku'alofa to Ha'api then onward to Vava'u. You can also get to Nuku from Suva Fiji (not Nadi) via Air Fiji and I'm pretty sure there are direct flights to Vava'u from somewhere outside of Tonga as Vava'u is a popular expat/yacht locale. Perhaps from Fiji, I dont remember.