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:

My process:

1. sew the vest. Black velvet at the front and black linen on the back. It was lined with black linen.
2. I made a cutout to form a screenprint fo the original green layer.
3. I then screenprinted the green with green fabric paint.
4. I handpainted the rose with red, and the stamens with yellow fabric paint
5. heat set the paint with an iron.

Accessories included a fobwatch, oval belt buckle, pants from the opshop, Alivan's wand.
The final Outfit: