http://www.brew365.com/beer_dennys_bourbon_vanilla_porter.php
See also: http://www.brewersfriend.com/homebrew/recipe/view/204339
Original recipe uses Crystal 120L and Crystal 40L and dark munich. However, UK chocolate is apparently a lot darker so this is coming out the same colour.
Original recipe suggest 10ml per 473ml of beer and 2 vanilla pods for 7-14 days
If bottling 21L that is 444ml bourbon.
I think it might be a good idea chop and scrape the vanilla pods and soak in some of the bourbon (for sanitisation). Then add the whole lot to the primary. This is going to extract some flavour, so should probably be counted in the steeping time.
So: 2 vanilla pods scraped in 200ml bourbon for 2 days. Chuck the lot into primary for 12 days, then bottle with another 244ml bourbon and priming sugar.