Hi Paul
This is not possible natively out of a Mac graphics card. (Some PC cards can output both DVI & VGA using a relevant adaptor - but Macs have never been able to do this...)
However, I recently provided a solution for a client using a TV-One IT-V2-558 format converter (corrected)
http://www.tvone.co.uk/1t-vs-558-main.shtml (corrected)
Best setup is to use a DVI DA connected to the Graphics card to give you a DVI signal to the LED screen (and local monitor?). You can then use the format converter to convert to VGA for KVM distribution.
Alternativly - use a DVI KVM... Adderlink X-DVI
http://www.adder.com/uk/products/adderlink_x_dvi.aspx
I have one of the IT-V2-558 format converters in stock available (and DVI DA's / KVM's)
What resolutions are you using?
Do give me a call to discuss.
Simon