I am using the latest x264vfw_full_44_2851bm_44825.exe for the x264. I installed both the 32 and 64 bit codecs just in case Vegas 14 had a preference for one over the other.
Mine is The same, 64bit only, Vegas 15
Vegas 14 about the same speed
The only difference I could see is I had "Field Order" "Upper Field First" set. I changed it to "None (Progressive Scan)" like you and it ran 5 seconds faster!
That is not the craziest thing. Have a look at my CPU while I am doing this: 63% to 73% !
But as the guys on late night marketing TV say, "But that's not all! There is even more!"
I thought maybe on my PC, the rendered was somehow using my GPU. I disabled GPU support in Vegas preferences. CPU usage has gone up to a constant 73% and the render time is now 2:41! So the GTX580 GPU support for the on-screen graphics in Vegas was actually interfering a little in efficient rendering. Perhaps the more modern cards are causing even more interference?
You slow Xeon speed is a real puzzle. There is just nothing fast about my PC. It does have 32G DDR3 1333MHz RAM, but it doesn't change when I start rendering. I have a SSD C: drive that is at 0% during the whole render. I am writing to, believe it or not, a 500GByte PATA drive! Yup - none of that SATA rubbish! The CPU is not over-clocked - just 2.77GHz on each of its 6 slow cores.
I will try this on the other machine with the GTX1080.