Couple of quick points:
- Don't know of a ferry direct from Vieques to Culebra or vice versa. If you find one, please post it here.
- If there are 2 or more of you, consider taxi/car to Ceiba and fly from there. Would be cheaper than 2 flights from SJU to VQS
- Ferry has always been hit or miss, but now they only run the ferries that hold cargo/vehicles so it's slower and not often but sometimes you may not get on. I would be leery of making a financial commitment for something in Culebra if my plan was to ferry from Vieques-Fajardo-Culebra.
Others will know more... good luck. I am going to VQS this weekend I think. Not sure if I will ferry or fly, but if I fly I will take car to Ceiba and fly from there.