Rather bizarrely, I've just tried to find the article, and it's no longer online (so it seems). It was an interview with Pratchett called something like 'The Creator Speaks', and he described Vetinari along the lines of 'Machiavelli and Robespierre rolled into one' - I found the interview again last week, after seeing the film, but now when I google for it I just get my own post about it! Oh irony.