Though I've not chartered with Moorings in Neiafu, I've had to lean on them twice for assistance with projects (that got them nothing) and they've been wonderful.
I'm not a big fan of Vava'u, as popular as it is. I'd rather spend my time in the Ha'apai group, but to each their own. I don't know what Moorings' policy is concerning travel to Ha'apai.
Neiafu has a nice market. And, a well equipped hardware
store as if right out of Mainstreet USA! As said, fresh food is inexpensive.
Cyclone season is November through May, so be careful.
If you go, there is a Hunga Tonga, a new volcano that's broken the surface. That would be a great visit!