Go to Canadian Tire and buy a small file (the triangular type) and a can of flat black paint.
I think you can figure out the rest on your own.
edit:
this is the one i use
http://www.canadiantire.ca/AST/brows....jsp?locale=en
along with a regular flat file if needed.
edit 2:
unless you meant the VFC sight is too loose no matter how much you tighten it..? In which case you just need some electrical tape to "shim" the inside surface of the sight (the angled part).