"Vegas Pro™ 11 leverages the processing capabilities of modern GPUs (Graphic Processing Units) using the industry-standard OpenCL™ framework. " Grab a laptop with a GPU that's good at openCL acceleration. GPU will be more power efficient for video edit than cpu and faster.
The Dell you spec'd is certainly strong however if you really want 5 hours..."battery life is underwhelming, but that’s the only real weakness in the Inspiron 17R SE." http://www.notebookcheck.net/Dell-Inspiron-17R-SE-7720.76609.0.html
Suggest you assume an intel i5, then look for long battery life with a dedicated graphics card for your vegas edit/render. Check vegas for recommended memory size.