With regards to the weather, the local 'Christmas Winds' can be very blustery between mid- December thru January, although as toddlich correctly mentions there are plenty of places to dive, no matter the conditions, especially if you are a macro / critter person. (Although my personal favourite time to dive is April / May / June / July).
Getting to St. Vincent is a little tricky, but thats one of the reasons that the diving here is so good... there are direct flights from Puerto Rico (sometimes that flights will stop at St. Lucia or Canouan, but you and your stuff stay on the plane for the 15 minutes it takes to add / remove additional passengers). You can also get here via Barbados. On the way home there is an early flight from Grenada to Miami, the LIAT flight from St. Vincent leaves at 06.30a, to get to Grenada to meet the onward Miami flight which means that you can link with other flights and get home in one day if you live further afield in the US.
Accomodation... Young Island is nice, but it is quite costly, and yes the noise from the nightclubs is a pain, especially at the weekends. I have had guests staying at Young Island and this was an issue for them. A 3 dive day and couple of extra rum punches help
Mariners offers comfortable, a/c rooms with internet and flat screen TVs, or take a look at Beachcombers, this hotel has a selection of rooms from very basic to very plush pent house suites with full kitchen, top floor giant balconies with lovely views over Young Island Cut.
Picasa Web Albums - Kay - Beachcombers
Just to give you a flavor of what you can expect to see underwater here are a selection of some of my favorite shots that I have taken in St. Vincent since Christmas.
Underwater photos
With regards to diving in Bequia, if you can schedule a day its very easy to get there by ferry, the Bequia Dive Adventures folks will pick you up from the ferry dock if they know that you plan to dive with them, alternatively why not look at doing a few days in St. Vincent followed by a couple of days in Bequia? Very do-able if you don't mind changing hotel after a few days.