I have no charter
experience in that area, but we cruised there extensively in our own boat. The Moorings has a base in Marigot Bay, St. Lucia, and another in Canouan, in the center of the Grenadines. You might want to check them out for a one way charter.
When heading south from St. Lucia, we would sail from Rodney Bay down to Soufriere and the Pitons, clear out there, and spend the night on one of the moorings available in the area, then depart early the next morning for Admiralty Bay in Bequia (55 nm), arriving in the afternoon in time to clear in before the Customs
office closed. We chose not to stop in St. Vincent. Some enjoy the island, some don't, so you need to make a choice. Everyone
enjoys the Grenadines.
For a week or 10 day charter, I'd limit your southern range to the Grenadines, and skip Grenada
. There's more than enough to keep you happily occupied there, and it burns up some of your charter time to get there if you begin in St. Lucia. If you have two weeks, reaching Grenada is possible without pushing too hard. You would clear out of SVG on Union Island and clear in to Grenada either on Carriacou (spending a night in Tyrell Bay) or at one of the Ports
of Entry down at the south end of Grenada after a longish day's sail (45-50 nm). In my opinion, the southern Grenada area is a really great for long-term cruisers to hang out, but compared to the Grenadines, I don't think it's as good a destination
for a charterer with limited time to spend.
p.s. Many charterers start and end their charters in Blue Lagoon
on St. Vincent, heading down to the Grenadines for the bulk of their charter time. One benefit of that (or chartering with the Moorings in Canouan) is that you don't have to spend time clearing out and clearing in.