Don't listen to >>502990
He simply isn't aware of mentalray's capabilities... Mentalray has all the features of Vray and more. It's one of the oldest and longest developed ray-tracers around. You're just not using a proper lighting setup, this is an issue regardless of the renderer, wouldn't change just by using Vray.
You need to turn on Final Gathering in the rendering settings. You need to use Area Lights. And perhaps even create Image Based Lighting using the button in the render settings (you'll plug a downloaded HDRi image into it, get a nice outdoor one.).
There's a lot more than that involved with getting a great lighting setup and tweaking rendering settings... but that is the bare essentials to get something halfway decent in a biased renderer.