As discussed in another thread (LINK), putting a dart in tartan fabric skews the alignment of the woven stripes. Here's a ladies vest that shows it in the extreme (ladies generally need more front shaping than men )



But there are men's vest patterns without darts. Here are two vest front pattern pieces, one with dart and one without.

WITH:


WITHOUT:


It's subtle, but the one without darts has more shaping along the side seam. Also not shown is the back pattern piece. With darts, the back is one piece (no seam up the center back). Without darts, the vest has a center back seam that contributes to the shaping.

Your body type determines how much, if any, shaping your vest requires. If you are only ever going to wear it under a jacket, the addition of a rear belt can help cinch it in without putting permanent darts in the front to mess up the tartan alignment.

Hope this helps shed some light on the discussion of tartan vests!