I prefer to wear a belt most of the time with my kilt (even with sweaters) as I have no hips and even a well fitted kilt feels in danger of riding down. If I were to wear a waistcoat I would most likely still try to find a way to wear a belt with it for that reason alone (feels heavier in the waist and a little more secure) as well as to more visually define the waist as a skinny man in a kilt shows almost no figure. It is up to the individual really (as with most kilt issues) as to what looks appropriate and feels comfortable though you should compare and search for pictures of those wearing waist coats and see if you can find an example of how to do it to spec so to speak.