A Google search for Redshift Vray takes me here:
To use Octane and Redshift you will need a GPU that has CUDA-Cores, meaning you will need a NVIDIA GPU. VRAY-RT additionally supports openCL meaning you could use an AMD card here.
The best bang-for-the-buck NVIDIA cards are 1070 GTX (1920 Cuda Cores, 8GB VRAM), 1080 GTX (2560 Cuda Cores, 8GB VRAM) and the 1080 Ti (3584 Cuda Cores, 11GB VRAM).
As a "mobile workstation", the ideal choice is going to be something expensive with dual GPUs, typically referred to as SLI.
If you are serious about high GPU performance, you will go with a heavy luggable beast of a gaming laptop with two full-size desktop 1080 GTX or TI GPUs crammed into it, rather than the low power mobile (xxxM) versions.
This will likely cost you more than US$2500 for the laptop, and it won't be a small sleek device, but huge, thick, and heavy, with large air vents all across the back.
There are many examples to choose from, just do a search of your favorite web commerce site for laptop GTX 1080 SLI.