Hi Bobby. Yes the kilt waistcoat is normally shorter due to the kilt being worn higher on the waist than a pair of trousers. The traditional kilt is worn just below the bottom rib or even higher with a high rise whereas modern trousers are worn on the hip. This leads to the disparity between waistcoat lengths.

I would respectfully suggest that in order for everything to look proportional you need two vests. You may get away with something in-between but would you be happy? At the end of the day it's your choice. Wear whatever feels right for you.

Don't forget pictures if you make one or two.