Either way won't be too easy... line loss is the big issue. Over VGA, it's analog signals becoming undermodulated, over DVI (which is what the HDMI would revert to from a DVI source) it's digital data loss.
And to fix it - it's the same problem in reverse - how much do you boost voltages? If you underamplify the VGA signal, your R, G and B signals will be low at the far end (not a HUGE deal)... but so will HSync and VSync (which IS a big deal) if you overamplify, you'll have blooming colours and sync issues. A pro would calculate and measure line loss - as an amateur, it would be a lot of guess and check.
For long DVI runs, most often I've heard of electrical to optical conversion, but I've never worked with the installation of one. Can't be any help there.
Bren R.
