Ok, my plan now would be as follows:

12/26 - Arrival at SJU (late at night)
12/27 - Explore the rainforest and Old San Juan
12/28 - Explore the rainforest and Old San Juan, fly to Vieques
12/29 - Vieques, beach, biobay
12/30 - Vieques, beach
12/31 - Fly from Vieques to SJU and on to MIA

Are Vieques Air Link (VAL) reliable enough to build a connection on?
I must not miss my connection at SJU under any circumstances - but I guess with a connection time of 3 hours, I should be fine? Or do their flights often get canceled?

Also, I still don't know which hotel to stay at.
I need one more Starwood stay before the end of the year to renew my status, but the Sheraton Old San Juan has terrible reviews.
I could stay at the Sheraton Puerto Rico Hotel & Casino, but that seems to be further away from the action?

Or I could just stay with a different chain. I still have some Hilton points to burn, for example. And IHG is running a sale as well.
What would you recommend?

Hotel recommendations for Vieques are also gladly accepted.
Unfortunately, both the W Vieques and the Hacienda Tamarindo have minimum stay requirements I can't meet.