there are hdmi to dvi cables so if you can hook up the hdmi to the monitor then you can get a dvi to vga adapter for a few bucks off ebay or newegg.
but as for your specific card about doing two vga's i have no idea.
i have a geforece 9400 gt and i have it set up for 2 monitors, one is connected via hdmi and one via the dvi, currently not using the vga input.
i am sure this was of no help but there you go
Well i have a HD4850 atm and i have it set to use 2 screens and they are both using DVI to VGA and i have no problem, its been this way for over 6 months but with this HD6850 it says it wont support 2 screen via DVI to VGA, as you said you have 1 screen hooked up with DVI and other screen with HDMI so i guess i should be able to do so.
I'll try 11/29 Monday when it gets to my house