It' true if you don't buy the waistcoat with the jacket it may not match. So if both are black and don't match it looks like you tried to match them and were unsuccessful. So why not try a wool waistcoat that matches one of the brighter colors in your kilt? Perhaps a red or a yellow. Like the one featured here:
http://www.vestsbycharlotte.com/red-...html?cPath=2_8

I think it's a way to bring in some more color and the red always looks good at Christmas. Just my humble opinion.
Regards,
David