Check out this great article on optimizing Vray by Akin Bilgic. It goes very deep into explaining how and why your Vray scenes should be optimized. As someone who uses the "Universal Method" (as described in the article and Vray's help) a lot, it's nice to have a comprehensive guide to deeper optimization.
I've done a little bit of testing myself and have seen the improvements this way of working can deliver. The extra bit of attention to your settings can really make your scene look great. Even more advantages show up when using render elements for compositing. The elements are less noisy, so you can colour correct them even more without too much visible noise appearing in the final plate.
Combining the information in this article with the information in "Demystifying the Vray DMC Sampler" (by Toni Bratincevic) is a great way to get deeper into the inner workings of Vray and getting the most out of it.