It doesn't matter much which connection you adapt from. There are active adapters to get a VGA signal from DVI-D, HDMI, and displayport (they have to have an active D/A converter in them). For whatever reason it made the most sense to me to use the displayport... though you could technically do this from any of the digital ports...
might as well use one with the cable length you need already in place:
http://www.amazon.com/Cable-Matters-Gold-Plated-DisplayPort/dp/B005RISB3S/ref=sr_1_3?s=electronics&ie=UTF8&qid=1419704863&sr=1-3&keywords=displayport+to+vga#Ask
http://www.amazon.com/Plugable-Active-Converter-Cable-Supporting/dp/B00H4J5B9U/ref=sr_1_1?s=electronics&ie=UTF8&qid=1419705092&sr=1-1&keywords=hdmi+to+vga+cable
I couldn't find a DVI-D to VGA in the form of a completed cable, only as a short whip.
-------
As you know, there are problems with trying to use both the onboard GPU and a discrete GPU at the same time... The only time this tends to work well is on AMD systems with both AMD iGPU's and AMD dGPUs both active, as they all work with the same driver. Trying to run video hardware that requires two totally separate drivers to run correctly is problematic, as video drivers are heavily system integrated, and do not like to play nicely together. Sometimes it can be made to work with 3rd party solutions, but the problem is that it's never a truly "supported" configuration, so there are no long-term garantees that it will work.
Even using a D/A adapter cable is probably going to present some problems, as I'm not sure that the GPU will necessarily be able to communicate with the display and identify what resolution/refresh modes are supported, (may require manual configuration, I don't know).