I have several comments.

We did a one way from St. Vincent down to Grenada last January and I agree with CaptBligh's other comments.

First, I would spend the money on the sleep aboard. Depending on how busy the base is, you could get your paperwork and checkouts done that afternoon which could get you underway before noon the next day. This also provides an opportunity to get familiar with the boat systems and get things fixed properly before you leave the dock.

Personally I would not spend 2 nights in Union (Clifton) on the way up. You will have to stop there to clear in and stop again on the the return trip to clear out of customs. Think about Chatham or Frigate Bay there. Also, PSV is nearby and Mopion is a good day stop. We typically spend 3 days between Mayreau (Salt Whistle/Saline) and the Cays. If you have never been to the Tobago Cays, you may wish to plan more than 2 days in this area.

If you have the time, and the wind direction is favorable Bequia is very nice and worth the opportunity to tour the island, visit the turtle sanctuary, etc. Depending on the wind/seas, Friendship Bay (Bequia) can also be a great stop if it is not too rolly. We have only done this on a monohull in January and it was a bit uncomfortable. Personally I would not bother with Mustique, but that can be a matter of personal preference. The sail down island off wind from Bequia is great. Canouan is not a bad stop on the way up or down. You can hit Tyrell and Sandy Island again on the way back to Grenada