We chartered from Grenada and up into the Grenadines last summer (cheaper for us credit crunched estate agents) - and whilst I can't comment on winter season winds, I can say that all of our encounters with the boat vendors were positive - but if we didn't want anything when approached we greeted them with "hi, how ya doin'
" and only then politely advised that we had all we needed.
We had some useful anchoring assistance (although one inexperienced "vendor" took our buddy boat too near to the rocks where a wind shift would have been a problem) and the ice and banana bread deliveries in remote anchorages (eg Tobago Cays) were invaluable.
From what I have read before we went I believe that there has been a degree of education and improvement in customer relations, and that was confirmed by our experience.
Des in Clifton and Black Boy ("that's what my mother called me") in Salt Whistle Bay were 2 of our favourites, along with Sydney and Walter in Tobago Cays.
Have a great time, the Grenadines are a must do (much better than BVI IMHO),