Had such a good time in Culebra we never made it to Vieques.
If your cell phone doesn't work you can also check in w/ customs at the airport in Culebra. As mentioned above, PR
and VI are in different customs areas and you are required to check in when you arrive.
Culebra is a laid back place, very different from the USVI. The photo below gives you an idea of how the place works.
Dewey -- We found it easier to anchor on the west side of Dewey (Culebra's main town) rather than in Bahia Honda. The ferries come in on this side and create some wash, but you have good places to leave the dinghy on the main quay and a much nicer dinghy ride. If you anchor in Bahia Honda and the trades are up you have a wet ride back to your boat. On the Dewey side there is room for 6-8 boats to anchor, but stay to the left as you face the town. The ferries come in and turn around on the right side of the harbor and they don't like yachts in their way. The holding is generally good in sand w/ grass, but do check the set of your anchor with mask/snorkel. As you can see in the photo below it's fairly calm on this side of the island, even on a windy day.
Culebrita -- Definitely worth a visit, but be careful with your pilotage onthe way in and out. the photo below is looking NNE. Check your charts carefully, but my recollection is that you parallel the reef break on the left of the photo. The reef on the right isn't breaking in this photo, but you can see the outlines of it as light colored water above the masts of the two sailboats in the center. This place is good in settled weather but stay away if a northerly swell is running or the wind is up or is forecast to be strong. A walk to the abandoned lighthouse affords a good view and don't miss the 'jacuzzi' which is on the eastern side of the bay. Turtles nest on this beach off the sterns of the boats. There can be some swell here, the boat on the far left is in the best position -- behind the small reef. When we were there there were moorings in the bay, but I'd check them carefully as they looked a bit dodgey when we were there.
Have fun. We stayed a month. Don't miss Flamengo Beach and it's fun to rent motorbikes for a day or more. There are several rental places at the airport.
Finally, this is a good place to change crew. The ferry to Fajardo on the main island is so fast and inexpensive it makes no sense to take your boat there. Fajardo is easily reached from the San Juan airport and the air taxi ride to Culebra is not that much more than the taxi to Fajardo.
As for the trip back to the VI, it's only 20 miles or so. Pick a day when the trades are down a bit and enjoy the sail to weather.