There is no easy way to incorporate Nvidia graphics, some of the points have already been raised here, just to add my own views.
There is currently a ‘behind the scenes’ impasse between Apple and Nvidia, hence no current option.
I read this article recently, and I read something else (which I now can’t find) on either The Verge or ArsTechnica very recently which went into more detail regarding the impasse.
Anyway, the long and the short of it, is that there is currently no Mojave Nvidia web drivers available for for any Nvidia card.
Web drivers are available for Nvidia cards up to Pascal level cards (the 1080 series including 1080Ti and Titan XP) for High Sierra and you can use a script to hack an eGPU to run an Nvidia card.
The ‘easiest’ way to run a Nvidia card on a Mac is to buy a 2010 ‘Cheesegrater’ MacPro (they really should have stayed with that form factor, they were amazing!) and upgrade it and install a firmware flashed Nvidia GPU from macvidcards 
This obviously has drawbacks in terms of cost of investment, obsolescence etc
Alternative is to build a Hackintosh running the latest High Sierra, which you can spec as high as you want, admittedly its not a trivial undertaking but there are full build and install guides / sites online and actually once you get your head round it, its not too bad but you can end up with a machine that runs rings around anything Apple CURRENTLY offers (and cheaper). If you want a hassle free existence, best forget that one.
In regards to Vray, the current version supports GPU / Hybrid rendering with an Nvidia card, AMD cards are not supported (as they do not run CUDA), the upcoming version does not officially support any GPU rendering on MacOS, this is partly because of the lack of drivers available for MacOS under Mojave and also no CUDA drivers (which Vray uses for GPU compute).
In addition, Apple’s move away from OpenCL to their own Metal Compute Shaders is another factor (that will eventually effect all other renderers that use Open CL for GPU rendering (Indigo render etc).
From what I’ve heard from within certain developer teams is that Metal Compute Shaders are currently not viable for GPU raytracing, if they were it would be considered but they are not so go figure.
It is a very difficult time for anyone that wants GPU compute capability (for rendering, photogrammetry processing etc) on MacOS, there is no clear future path, no CUDA, OpenCL being deprecated, Metal Compute still immature (maybe)
I personally find it insane that the builders of the worlds largest (and arguably best to their users) creative technology hardware platform does not and will not (currently) incorporate the market leader, and arguably state of the art GPU hardware to help drive that platform. Not all of us MacOS users are video editors, we are artists, architects, engineers, artisans, game designers, musicians and scientists. Currently the best GPU compute capability is delivered by Nvidia and looks to be for some time.
I hope 
that the new MacPro offers those options - it is said to be built from the ground up as modular. I hope that Apple pulls the rabbit out of the hat with a technology partnership with Nvidia (I severely doubt this as AMD is a valuable partner and Apple’s Pro apps are built around AMD hardware acceleration).
I have been a lifetime Mac user (even used to be a Apple service engineer) but last year I seriously considered moving to Windows purely for the GPU compute future and Vray (actually I was using Maxwell Render at the time, which has GPU rendering on Windows). Anyway, I didn’t and ended up building a Hackintosh (which I am very pleased with).
But yeah, I’d probably hang on a bit and see!
I forgot about Vulkan. sorry. There is an unsupported (by Apple) Vulkan layer that runs on top of Metal (Vulkan VK, I think), that does offer compute capability but thats miles away from an implementation or software support.