Alfred or Yang
All MacBook Pro have discrete cards built in, and mine has a Radeon 6750M. The only ones that don't are the ones that start with and only have Intel HD graphics listed in their specifications.
This extra card allow you to drive an additional monitor (or your VGA projector in your scanner), but you have to use a Mini DisplayPort adapter to access this ability. In my case the MacBook has a Thunderbolt port, and the right adapter will get you a VGA output!
This is the screen you need to see in Windows 8.1, and it's accessed via PC settings>PC and devices>Display. As long as you see a set up like this, display 2 being 800x600 60Hz (set under Advanced Settings), then you are good to go.
Windows8_display_settings.jpg

Also as Hugues said, you need to use the Extend these displays setting too.
Your instructions were slightly confusing, more so for me, as I didn't understand the operation of your scanner. However, now that I do, I know I can use an older 27" iMac with your scanner, as well as a later version, another screen, plus your scanner in a daisy chain!
My next task will be to get a legacy iMac to work, and we'll see what else I'll manage too.

Hope this helps you guys
Here's a test scan
torsion_bar_cover_render.jpg
Mike