We were recently in Italy and bought a large piece of parma ham, which we'd like to use over a protracted period. It is vacuum packed at the moment, what is the best way to preserve it once opened?
To best preserve it's texture, especially that slightly moist part a good ham is supposed to have, I've found wrapping cured meats in plastic foil and keeping them in the fridge at a temperature where there is little condensation (not extremely cold, but you have to know your fridge) works for a decently long time (couple of weeks at least, if not months).
However things wrapped in plastic in the fridge tend to get moldy over time, there is no escaping that. And dried salted meats are after all a product of ancient preservation techniques. So if you really need it to last storing it in a dark and a bit chilly area, wrapped in a clean dry cloth or paper it should also stay good for long, although it will dry up some after it is cut and not vacuum sealed.
Last option is to wrap it in some paper and leave it on the bench. The paper should protect against dust and moisture, but at that point I would suggest that just eating it is a better way of storing it if it is a really good piece you spent some effort in acquiring.
I can't comment specifically on parma ham, but I find that in general for cured meats and firmer cheeses that wrapping in butcher paper and then placing in the fridge works quite well. It's possible that you might have a little bit of dried outer layer that you have to deal with, but it's less likely to go moldy on you, making you want to throw away the whole thing.
If you don't have butcher paper, I've also had good luck wrapping blocks of cheese (I admit, haven't tried this one with meats) in a paper towel, then placing it in an unsealed plastic bag. This helps to wick away any surface moisture, and the open bag slows the evaporation but keeps you from accumulating too much moisture. (note that the 'meat drawer' on many fridges works like the open plastic bag -- it holds in a bit of moisture to keep things from drying out too fast)