Sirius wears a few different frock coats of varying lengths. Dave found a blue velvet suit jacket as a substitute.
I found a copy of a BBC documentary on the costumes from Harry Potter 5. This discusses this very vest and how it was made (which was similar to my thoughts on avoiding embroidering the whole vest, so I was very happy). This can be found at http://www.youtube.com/watch?v=zdSrkFNLzrE
The black velvet was essentially screenprinted to give a basic print, then the rose and stamens were handpainted on later. This is the end result of their process:
Accessories included a fobwatch, oval belt buckle, pants from the opshop, Alivan's wand.
The final Outfit:
(c). K.Carlisle, 2008.