I looks to me like a late 1700s style 'sleeved waistcoat' with a few Highland touches, you could base one on something like this; http://jas-townsend.com/product_info...roducts_id=833 Just add the cuffs, some trim, and a mess of buttons...

I always look at these paintings with a mental grain of salt, how much was from life, and how much artist's fantasy, we don't know.