Assuming you aren't wearing a jacket -- only the vest -- I'd personally be inclined to go with a shade of gray matching one of the many in the tartan itself; this should give you quite a range, and you could even have a couple ranging from lighter gray to charcoal (?) to suit any given occasion.

If you want something a little brighter, pretty much anything could work, though I'd think warmer tones (reds) in particular would be quite striking.