Thanks for the new opportunity for controversy!
I wonder if some sort of strap-and-button arrangement could be installed to the bottom of the waistcoat which would fasten to a corresponding holder attached to the kilt? For instance, buttons could be sewn to the inside of the top of the kilt, near the first and last pleats so as to not pull on the aprons, and straps with buttonholes could be sewn to the inside bottom of the waistcoat/vest, and the wearer could then attach the two, thereby holding the waistcoat down and the kilt up at the same time. These appliances would not be seen, and if done right, I believe it might solve a lot of people's problems. This system could also be used for pants/trousers/britches, as well, attaching to the buttons that some pants already have for suspenders/braces.
If anybody patents this system, I want half the royalties.