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It looks to me, after seeing the whole image to be a pocket as I first mentioned. Looking at the whole painting, you can see the colors and patternd repeated on the opposite side. I don't know if its some hybrid waistcoat where the brocade was tacked onto waist length plaid waistcoat, or if the plaid waist length vest is worn over a full brocaded waistcoat (which is what it looks like to me), or some invention of the artist all together. If you magnify the image you can clearly see that the brocde appears on both the upper and lower edges of the belt. You can also see that the plaid waist length garment lays over the belt, appearing, to me, to be garment sepereate from, and being worn on top of, the brocade.

I'm not saying it looks good, or appropraite, or that it makes any sense. But it would not be the first instance of someone of the period wearing more clothes than seems necessary.

It's always dicey to read too much into a work of art, it is a painting, not a photograph.