It's been indicated in another thread (Matt even posted some pics), that as long as the difference in sett size is small enough, it is quite doable to have a different weight fabric in the vest, as in the kilt. If the heavy weight kilt has an 8 inch sett, and the medium weight vest has a 7 inch sett, few will likely notice, particularly if the vest is cut on the bias.

But what is more important to consider, in my opnion, is that the dies used between each fabric match. For example, I see that the yellow particularly, differs a bit between the Locharron heavy weight Strome fabric of my kilt, and the Lochcarron medium weight Riever fabric of my tam-o-shanter and waistcoat, all in the Buchanan Modern tartan. In the lighter weight fabric, the yellow has more of a gold tone. If I wore the vest with a suit but no kilt, or the kilt without the vest, no one would notice. But worn together in sunlight or in a well lit room, it's likely noticeable if you look closely.

So I would say to get best possible match of the colors, get both the kilt and the waistcoat made at the same time, from the same bolt of fabric.