I do like the idea of a tartan vest. The trick is going to be to find something that doesn't clash with the kilt you have, but at the same time isn't too dark. You want it also to stand out against the jacket. My first inclination, just looking at the avatar was for a green-based tartan with some of the same secondary colors and maybe a red pinstripe, but it would have to be in ancient colors I think to not hide in the shadows.