My personal understanding on the matter is that regardless of the licence you hold, you need to have a Medical accepted by the national Authority of the place you are going to fly in.
If you can pass a JAA Class 1 I don't see the reason why you couldn't get another medical, the JAA Medical requirements are for sure higher than the FAA (US) medical requirements and in my own experience quite acceptable.
I think both FAA and JAA allow the pilot to wear corrective lenses if they can correct his vision to an acceptable level.
Maybe if you ellaborate a little bit on the nature of your problem there might be other posters with more insight on the matter.
-Polorutz