Just seen your other posts.

Thanks for the input, Nick. 
After much more research, I've come up with this list of components for my new system. 

I've decided to use the 6-core i7-6800 now, but leave the door open for a possible upgrade to the 10-core i7-6950 in the future.  Maybe in a couple of years the price will come down.

   Chassis:              BeQuiet Dark Base Pro 900
   Power Supply:     EVGA SuperNOVA 1300W   
   MoBo:          Gigabyte GA-X99-ULTRA-GAMING
   CPU:           intel i7-6800, upgradable to i7-6950 
   CPU cooler:    Corsair H115i
   RAM:           64GB total in 4 16GB DIMMs, upgradeable to 128GB
                     G.Skill Trident Series 3200, CAS latency 14
   GPU 1:         AMD RX480 8G (for monitors)
   GPU 2:         ***TBD***    (for render accelleration)
   Storage 1:     1TB Samsung SSD via M.2 (boot drive, OS, applications, plug-ins)
   Storage 2:     2x 500GB SSD via SATA III (files for current active project) 
   Storage 3:     2x 4TB HGST Ultrastar in RAID 1 (files of archived projects) 
   Storage 4:     LG BluRay Rewriter (DVD/BD mastering)
   Misc:          Multi-interface with Card Reader (SD/microSD/CF), USB ports, eSata, 1294 
   
So Vegas Pro can only take advantage of certain GPUs for rendering?  
And evem then, it only helps certain Codecs? 
Will more GPU acceleration be coming in the future? 
   
   

I wonder if there are plans to fully support (utilize) workstation graphic cards in the next version of Vegas (15)? I'm thinking about the Radeon Pro WX 7100 specifically. In another thread here there is at least one account of a previous Vegas version (13) not supporting the 10-bit output, but then another user says Vegas supports the output on his Quadro. The 10-bit output support is the primary reason to fork over way too much money for one of the workstation cards, considering other specification numbers like memory bandwidth just does not compare in their favor with consumer-grade gaming cards around the same price point.

Squeezing out all of the acceleration possible from the top tier workstation cards should benefit those of us using consumer-grade cards as well, as long as the feature is not limited in some way.

Perhaps the support I'm looking for is more in the codec developers' hands, since that is where the acceleration is really needed. Until the codecs are written to take better advantage of the GPU(s) something like the i7-6950X is our only real hope for more encoding and decoding performance.

In another thread here there is at least one account of a previous Vegas version (13) not supporting the 10-bit output, but then another user says Vegas supports the output on his Quadro.

Links to those threads would be very useful. They may help to settle this unresolved question.

Links to those threads would be very useful. They may help to settle this unresolved question.

The discussion between Megabit and Wolfgang toward the end of that thread is the one I was referring to.

My perception that GPU rendering in the codecs is lacking is due in part to an admission in another thread (here) and my own experience with Vegas 13 and a Radeon R9 290A. Monitoring GPU utilization with Microsoft:Sysinternals' Process Explorer (Properties->GPU Graph for Vegas executable -or- the relevant subordinate executable) while encoding in Vegas shows the lack of GPU utilization during rendering, although this was already admitted in the linked thread as being due to legacy code in the codecs. Hopefully NeuLion has updated their GPU code for their codecs.

TotalCode Studio has "improved 4K HEVC and HEVC SABET transcoding performance," so perhaps Vegas 15 will as well.

When utilizing the HitFilm Ignite Pro LUT plugin my CPU utilization on my i7-5820K (w/all cores @3.7GHz) caps at about 40% (VisionColor LUT plugin was consuming up to 40% by itself). The R9 290A GPU core utilization caps at about 3% and a maximum of 3GB out of 4GB VRAM is used, averaging around 2.5GB for my test. There is high GPU VRAM swapping observable, but this card has a 512-bit GDDR5 interface. With I/O transfers staying minimal throughout the encoding process there are no obvious hardware bottlenecks where more hardware would improve the encoding time.

For those with >4GB VRAM does Vegas 14 utilize more VRAM, besides the case of heavy OFX usage?

It would appear processing larger chunks of data at a time through the GPU could improve performance, but that is a flat out guess.

NOTE: I used Sony's XAVC-Intra codec @UHD resolution during the test since that family of codecs is quite GPU optimized.

Since GPU encoding acceleration appears heavily GPU memory bandwidth dependent, the Radeon Fury X and Radeon Pro Duo must be the best budget-is-of-no-concern options due to the use of HBM, even with things as they stand now with both Vegas 13 and 14. Plus, with the massive amount of stream processors, imagine if the Vegas 15 codecs actually took full advantage!

Oh, and I have been reading up on the local lore concerning this subject. Kudos to the knowledgeable user base!