The most irksome thing about the setup is that I had the hardest time getting both the DVI and VGA displays running as an extended desktop. Until now!
I still haven't worked out the vagaries of getting X configured, but once I'm at the desktop, it's a simple matter of
xrandr --output VGA --autoSpecial thanks to the folks at intellinuxgraphics.org who led me down the path of multi-monitor bliss!
xrandr --output VGA --right-of TMDS-1
Update: In tweaking xorg.conf, I realized that it's necessary to add the clause Virtual 3200 1200 to Section "Screen", SubSection "Display". Without it, xrandr gives errors about max screen size.