I agree with the advice given so far. As they have said there are various studies which have indicated that vitamin C has a protective effect and it is very safe. It seems that taking it for a period of 50 days is supposed to be helpful.
Have to discussed your higher potential risk of FRPS with the treating orthopaedic consultants and/or physio therapists who may be involded in your care? One risk for CRPS is a period of immobilisation such as with a cast or brace. You didn't say whether you have had the wrist immobilised. If you have, highly recommend discussing this with the ortho/PT as a matter of urgency to see whether (a) it is really necessary; (b) the risks of keeping your wrist mobile outweigh the risks of compromising your suspected hairline fracture; (c) whether they can look at an option to keep the immobilisation to the absolute minimum period even if this is shorter than might be ideal given the risks it creates for you; (d) whether there is an alternative type of casting, brace or support that could be used to protect your wrist but to give more movement and less immobilisation and (e) whether there are wrist and hand exercises you can do several times a day to make sure you are keeping all of your hand, digits, wrist and elbow as mobile as possible without compromising the possible fracture.
It's critically important that you keep a close eye on the tightness of any cast or brace - that may or may not be a risk factor for CRPS. Its not that clear whether the pressure from a too tight cast contributes to CRPS or whether its the swelling from the CRPS that causes a cast to feel too tight. If you notice it feeling too tight or excessive swelling don't hesitate to go back and seek advice immediately. Then run through all the stuff I mentioned above to see if there are any other options. Similarly, if you find your fingers turning blue, purple or black as soon as you drop your hand down, that could indicate a problem so seek advice.
Finally, it is essential that you have sufficient analgesia. Another potential risk factor for CRPS is insufficiently controlled acute pain. If you are able to, given your higher risk of CRPS, you should ask your ortho or GP to consider a neuropathic pain drug on a prophylaxis basis. A drug like gabapentin can be given to pre-empively control or 'dampen' any neuropathic pain for the duration of the healing period. You should also make sure you are not just putting up with any pain from the injury - this is not sensible. You may need to have either a larger dose or a more powerful painkilling medication than a person with the same injury who doesn't have your CRPS history, just to try to minimise your risk. You may also need to take a painkiller for longer than someone else.
Hope that all helps. Sorry it's a lot but these are all the things you could look at and discuss with your doctors to try and minimise the risks. Theres not a massive amount you can do to prevent CRPS but these are the things which might reduce the risks based on things that are thought to contribute to triggering the condition.