What type of processor are you using to have 16 cores?

It probably does not have a real limit, but there is always limit of diminishing returns when adding more cores.

From what I have observed, the Vegas video engine only uses cores fully if you have enough video tracks as input for the rendered output video stream. Render in this case has nothing to do with "render as" which is encoding a file.

For example, I can have a single video track with a 30Mbps AVC GoPro file and only see 15% or so CPU utilization even if it stutters. If I have four video streams then I see values approaching 100%. This on an i7 4 core CPU with hyperthread, which windows shows as 8 logical cores.

So the more parallel video streams (sync'd cameras) you have the more cores Vegas will use to render the video stream.

As for "render as" file encoding. Each encoder is different with its own limits.

"So the more parallel video streams (sync'd cameras) you have the more cores Vegas will use to render the video stream."

This is really quantum rocket-science: I have 8 physical cores - and when rendering 1 (one) single video track with full HD 24 Mbps AVCHD 50i no FX no anything - to 31 Mbps Mpeg2 50i full HD the 2 quad Xeons are running at 85-95% utilization (GPU disabled).

The yield is rendering 1 minute timeline in about 49 seconds.

If I render for previewing and audio syncing - that is same timeline but the output is Mpeg2 at 1280x720 at 11 Mbps 25p for the PC monitor - then 1 minute Timeline renders in about 24 seconds - and still utilizing the CPUs at about 90+ percent.

It took some tine to figure out that this hardware + Vegas 12 + the Main Concept Mpeg2 encoder has a speed peak at about Max rendering threads = 5 and Max Preview RAM = 22 MB ... but it is really convinient that the many small daily temporary renders for previewing runs in about 2-3 times real timeline speed.

This is really quantum rocket-science

As I stated, I was not talking about encoding a file. "Render as". This involves two different and separate "renders". Vegas rendering the video stream from the multiple inputs and the file encode render.

File encoders typically have many threads doing analysis for compressing the video stream.

What utilization do you have when playing back that single video track?