Vieques weather patterns have been changing according to my observations. This is not scientific. Used to be that we had a clearly defined rainy season and dry season. The rainy season was supposed to be from August to November, which overlapped the hurricane season. Then the dry season was supposed to start in December and last thru the summer months. Well, not the last couple of years. It seems like the dry season has expanded to the point that the whole region suffered a drought during the summer and fall months of 2015. The rains came late and not as intense as I remember. Of course this does not mean that it will continue to be like that.
Other than that you will need to know that it is always warm in here according to the northern standards. Although we here find it so cold in the “winter”, we seldom venture in the water.
Nevertheless, it has been a tourist delight. No hurricanes, not much rain… Enjoy it.
For more information check this page. Note that the data presented in this site is for San Juan and not for Vieques.
This other site has data from Roosevelt Roads, closer to Vieques. Weather Underground There is a tab for historical data.