To match the tartan on the front align the center line of the waistcoat (usually marked as a button or buttonhole line on the pattern) straight through the corner of the sett. Cut one side out then place it right sides together with your remaining tartan and line it up exactly at all edges pinning it in place so that it doesn't move then cut the second piece out. You should end up with perfectly mirrored front pieces. make sure you mark the right side of each piece because you can very easily lose track of it when everything is cut out at crazy angles.

That is a wonderful brocade waistcoat I was going for that look when I made my regency outfit but the waistcoat came out a bit large so I wore my doublet buttoned up and went without it.