Quote Originally Posted by M. A. C. Newsome
So long as you are having the vest made by the same company, and from the same cloth, I don't think you'll have a problem.

If it is a quality item, the cloth should be a nice strong black, and it should be pretty uniform from one garment to another.

The only instances I can recall seeing jackets and vests that were off-color were cheaper ones coming out of Pakistan.
Alternatively, if you can tell which company made the jacket (Gaelic Themes, etc) than you can probably find a better deal online for the waistcoat. My local shop sells the black argyles for $399 and the waistcoats for $150. Meanwhile I got my jacket and waistcoat online for $380. Sometimes the local shops are just a wee bit pricey.