I wear waistcoats with my kilt all the time, as I'm a big proponent of the modern/casual way of wearing a kilt. I have loads of waistcoats that I've bought from vintage shops in Camden. I've found that vintage waistcoat are made of better materials than modern ones and tend to be shorter in the front, meaning there are no issues with my sporran. Plus, vintage = cheaper. And you're safe in the knowledge that no-one else will have one like yours (probably).