Points are good for < 1 week stays. In general, the earlier the better. Many book 12 months in advance. I am not aware of a comprehensive when to book by resort type chart, might be nice. However, in many properties, easier with points. But not all. Your best bet is to poke around mvci.com and pretend search for the resorts you might be interested in while it's still summer. What is available this short term? If the place(s) you want to go are not, then, it's going to be better to book much earlier for next year.
As of right now for example, the only Caribbean resort available via points for August 2018 for 5 nights is St Kitts. 4 nights would add Aruba Surf Club. If I look however for June 2019, I see all but the Ritz available.
A trick to conserve and make better use of your points for 5 days is to not arrange a Fri or Sat stay. 5 days without those 2 days is much cheaper points wise, depending on the resort and the season. 5 days St Thomas for example in June 2019 is 1875 if you skip those days. But you can do Aruba in Sept for 2/3 of that too.