I wouldn't go for a Dell Vostro or a Dell XPS.
Dell has recently launched a new line of Latitude E series notebooks
(aimed at the business class, but also very appropriate for students) which are lightweight, sleek have superior battery life - for example, a review for the Latitude E6400
showed that battery life with a standard 6-cell battery was more than 6 hours, and the E6400 is a 14.1" notebook in the mainstream range. The E4300 (13.") and E4200 (12.1") are in the ultra-portable range, and you can be sure to expect more than 6 hours battery life.
Most models come with an LED backlit display and an adjustable backlit keyboard. You'll find models with 15.4", 14.1", 13.3" and 12.1" size displays.
Here are some informative videos about the new Dell Latitude E series notebooks. The webcast in particular gives you a lot of useful information such as battery life, which I'm sure you'll find very appealing: Dell Latitude Webcast