If you want to make a Buffy-verse style vampire, this page is a good resource. http://dfrpg-resources.wikispaces.com/Hellmouth+Vampire
If you want to make a Red, White, Green or Black Court vampire with a soul, I'd rule that you'd need to have enough Refresh to have at least 1 left over to represent your free will. This is easiest to do with White Court, but only possible with Red and Black Court in higher powered games.
I suspect that Thomas may have enough Refresh to still have a soul, since he can choose to act against his nature in some of the books, and even describes his later descent as an easier path rather than outright succumbing to his nature
, but this would be speculation.