EDIT: "en" can sometimes be used for regions, countries, etc, but cities use "a".
It's quite funny, you know. Not only back in the Jurassic, when I was in school, but much more recently, using "vivre" instead of "habiter" was absolutely wrong, according to the teachers and books I was exposed to. "Je vis a Paris" would rate a scolding. Every time I see it used here I want to laugh. Lovely.