I think it should look fine, I do try to match up what I'm wearing most of the time. However there are times, and more recently I've been of the mind that we put far too much stock in matching everything when it comes to Highland wear, as you said yourself it is not a truly formal event I think the gray waistcoat would work, the upside is you need not worry about a belt black or otherwise. Of course as I said in my original post I'd like to see a picture of the proposed ideas as well. One other thought that comes to mind is the Gray Waistcoat will help to pull your lovely dress sporran into the outfit, were you to wear both.