Hi Donald, not sure when you are planning on flying in to St Vincent, but the direct flights from Toronto stop either in April or May so plan accordingly. I would look into doing a one way trip from St Vincent to Grenada (additional charge of about $1K for this option, also additional flight complications as it is two one way tickets so that is also something to consider unless you fly back to St. Vincent), the ability to run down and not beat back may make up for the hassle of checking in to Grenada on Carriacou. If not you could definitely spend a week sailing in the Grenadines. As sunset sammy mentioned, Bequia is a definite must. You could do: Bequia (2 nights), Mayrea (1 night), Tobago Cays (2 nights), Union (1 night), PSV. You might even be able to stop at Petite Martinique as there doesn't seem to be the same restrictions about dingying over from PSV ...
In terms of charter company, We are chartering from DYC in May this year in Grenada a catamaran (brand new 47' Saona with 4 couples) but did however charter a 45' monohull last year through them. There were some issues (fridge not working, battery issues) but I think that will be the same for all of them down there. We booked in September last year for this May and had no issues with availability.
Our trip is 6 weeks away, will have more insight after that as I have never been past PSV (only sailed in Grenada last year for our sailing course.) Can't wait!