The 38 year old actress is wearing Christian Dior Fall Winter 2014 off-shoulder dress on the cover that was snapped by Mario Testino. This is her sixth Vogue US cover. (source)
Click here to view last year cover featuring Kate Upton, here to view 2012 cover featuring Hope Solo, Ryan Lochte and Serena Williams, here for 2011 cover with Penelope Cruz, here for 2010 cover with Blake Lively, and here for 2009 cover with Cameron Diaz.