Ok, so I bought "in bulk". This is a quart of clear vanilla that I purchased from a cake decorating supply store about a year ago. I would think it's still good, but will this yellowing cause problems of discoloring the icing?
The amount of flavoring you use in a batch of icing should not effect the icing at all.
The brown (real) vanilla will give you a slight off-white/ivory/candlelight color that many want
i recently bought a big glass jar of clear vanilla from puerto rico and although it was clear it had a yellow tint to it. i didn't notice it in the icing, the real butter colors the icing yellow more than the clear vanilla does. that stuff was awesome...i wish i knew where to buy more!
I agree that you should be okay. If you need to offset the yellow add a little violet to the icing.