Interesting. Here's why I started putting the waistcoat on it's own hanger. The waistcoat goes on me first, then the jacket. If the waistcoat is under the jacket on the hanger, I have to take the jacket off the hanger and find a place for it while I put on the waistcoat. The jacket has fallen on the floor a couple of times while trying to do this juggling act.

Just for the kilt jackets, I have put the waistcoat on a smaller hanger and hung it from the pants bar on the kilt jacket's hanger, and it takes up no more space than before.

Right at the moment, I only have one waistcoat for my regular suits. It does match a gray three piece suit, but it has to be shared with the other suits. My two kilt jackets do have matching waistcoats, but now that I think of it, I could probably swap the waistcoats for a different look.