I think the bottom of the waistcoat would normally cover the belt, if you are wearing one.
I have a suggestion for getting a waistcoat to work with your build, and it could be made to work with both kilts and trousers. The dress pattern companies sell patterns for waistcoats for men. Most of the patterns allow for several design choices in one pattern. One of the options involves an adjustable neckline that allows you to control the length of the waistcoat. Basically, the front pieces of a backless waistcoat are not sewn together at the neckline. The top of each front piece narrows down to a strip; by varying where you connect the two strips, you control where the bottom of the waistcoat falls. The strips can be connected with Velcro, buttons, or a buckle.
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