Waistcoats meant to wear with a suit will generally not work well with a kilt, I've found. Modern waistcoats are too long all the way around. Those made to go with suits from the early 20th century (say, 1920s-1940s) are the correct length in the back because of the high trousers worn then, but they usually (not always) have a long, curved 'swallowtail' in the front, which gets in the way of the sporran. Kilt waistcoats for daywear usually combine these two, vintage length and a modern, cropped front.