Dave, there's not much you can do about it, your processor is already too fast for the encoding preset you're using.
If I remember correctly, you're saving the work in Vegas using some CRF preset... that means constant rate factor, as in the quality of the video is constant from frame to frame.
The encoder doesn't spend a lot of time analyzing what happens between frames, what to remove or drop in quality in a way it won't be noticeable with movement, doesn't try to save bits by overcompressing some darker areas to have more bits for more lively scenes, this and that, psychovisual stuff, it just looks at each picture in the video, computes a quality factor for each small block in the image and then just goes down in quality if you set the CRF lower than the value of the factor computed for that area of the image.
Basically, it has less work to do compared to a normal video encode, where more programming code get involved.
On your old computer, the cores may be pegged to 100% simply due to the fact that the old processor doesn't have so many instructions like SSE3 to optimize the encoding, or simply put that old cpu can't do as many instructions per core as the new one can.
As for your laptop cpu, it's a quad core processor, not an eight core processor.
Hyperthreading makes the processor show 4 more virtual cores and the OS will assign threads to various cores. In your case, core 1 and core 5 are probably the same physical core, and you see those added up are close to 90% cpu usage ... same with core 3 and 7, they're at about 70% ... and core 2 is probably busing decoding the raw video (even though the video card decodes it, the video card driver still uses cpu feeding the card with the data and translating stuff between opencl/cuda and video card) ... so overall you have maybe 6-7 threads already working. It's just that if you're using CRF, simply put there's not much work to be done in the encoding stage.
Hyperthreading isn't full proof, same with the allocation of threads on cores - you may want to experiment with disabling it on your laptop, maybe this way the OS can put the threads more evenly on the cores, making each core work a bit instead of having 2 cores worked up and 2 idle.
I'd also argue that using the video card of your laptop to help in encoding may be a bad idea. Video cards in laptops generally are kinda slow, they don't have the bandwidth to transfer large chunks of data back and forth. Yes, the hardware video decoder of the video card may be able to decode video faster but the data still has to be copied to the video card then copied back and the video card driver gets involved... you should see if the overall encoding time decreases if you use software decoding.