Nice trip. The best provisioning is Grenada, but that'll be a little late for you! Stock up as much as you can in St Lucia, then if you get to the southern tip of St Vincent, around Blue Lagoon. There are charter bases there so you'll be able to get advice once you arrive. After that, you are getting limited. Bequia and Union are good for top ups. I've made the trip many times but not recently. Hopefully Zanshin will be along to give you more up to date info.

I don't think this particular forum gets much traffic anymore. Southern Caribbean or BVI will likely get you more responses.