I have had a computer before that used a Y cable to split a single display output into two - although that was likely DVI if I recall correctly. That did not duplicate - it treated it as two separate monitors and allowed extended desktop. Since VGA is analog tho, I don't believe it would work the same way. A splitter cable would simply duplicate the signal on both screens - you could not have separate views on each screen.
I'd still say if you have DisplayPort, just get a cable to convert that to DVI. Easiest answer.
"How do I get rid of a Trojan Horse?" -Sailor_Kakashi "Leave it outside the gates of Troy overnight." -Davel23