Not capped, just (maybe) not written to take advantage of these brand new processors. It's not "automagically" scalable you know.
How many cores Vegas uses effectively in practice is not an easy question to answer. You would need to test performance versus the "same" processor with less cores in a variety of situations. Without that testing I will guess 32 threads as this is what is shown in the Vegas 16 preferences. The Guru3D article only seems to have looked at one aspect (UHD render) and IIRC a Techgage article claimed the opposite. So who knows.
So granted there are improvements in memory architecture and bandwidth, and other system elements. But the graph does basically show how more threads helps improve render times. The bottom CPU is what 4 cores/8 threads and the top is 16 cores/32 threads.
What is not shown in the review are the details on each of the test systems. I find it hard to believe that each of the processors had identical ram quantities given the vintage of some the test machines. The 2600K might have had maybe 8-16GB of ram, while the 2950x could easily have a lot more due to the amount of memory channels.