We chartered for 7 days out of St.Vincent in 2006. It is a great place to sail. Some big ocean swells in the short passages between some of the islands and there is often a good breeze on. I recommend staying flexible and not trying to get to too many places. Cam's proposed itinerary looks good to me. Here is what we did:
Day 0: Arrive St. Vincent and overnight on the boat.
Day 1: Check-out and provision and sail to Admiralty Bay, Bequia. We had arranged provisioning through the charter company and then canceled and provisioned through a grocery store in St. Vincent I had found on the web. There was a mis-communication on this and it ended up causing a big delay in our departure. Because of this, we opted for the short sail to Bequia instead of the planned sail to Mustique.
Day 2: Explored Bequia from Admiralty Bay. There are beautiful secluded beaches on the other side of the island.
Day 3: Tobago Cays (get an early enough start so that you are not arriving too late in the day at Tobago Cays). You'll have some time in the afternoon to enjoy the Cays.
Day 4: Enjoy the morning in Tobago Cays and then short sail to Saltwhistle Bay, Mayreau
Day 5: Canouan
Day 6: Admiralty Bay, Bequia (you want to be close to St. Vincent for the return sail on the following day).
Day 7: Return to St. Vincent
One of the members of our party was particularly disappointed at not getting to Mustique. However, I'm glad we didn't try to fit it in. From what I could tell, it's not necessarily a short easy sail from Admiralty Bay to Mustique (usually most of it upwind and up current so your estimate is probably off unless you get lucky with the weather). Next time we go the Grenadines, we'll try to charter out of Canouan (Moorings has a base there) so that we don't have to cross the Bequia channel twice.
You might want to pick up these 2 books before your trip:
Sailors Guide to the Windward Islands
By: Chris Doyle, Nancy Scott
Adventure Guide to Grenada, St. Vincent & the Grenadines
By: Cindy Kilgore, et al
And there is some good info at this web site: