I Don't think (video) to USB is going to be particularly practical, or produce an accurate picture with less resource use then VNC or equivalent (because VNC can at least optimize the output by telling it to "draw a box of this color" and recreate a screen, rather then capture it. Similarly 100 megabit ethernet is faster then USB2 and gigabit ethernet is faster then USB3.
Have you tried enabling compression for your VNC stream and reducing the color depth ? This can make a HUGE difference in performance (particularly compression, and if you don't do "wasteful" things like have an image background). Also, you haven't advised what OS's you are running - there may be more bandwidth effective variants on VNC (like RDesktop, exporting your X Session)
You may be able to get a VGA -> H264 or MP4 Video capture and then play that back through your second monitor, but I see a few problems with this, including detail will be lost in the lossy compression, you will need more CPU/GPU to decode the screen as well as expensive hardware to encode the stream. I suspect by the time you are finished it would be cheaper to get a second monitor and long VGA cable.