Well physically there's some huge differences. The dvi plug itself has many more pins, so it's acts like the red/white/yellow plugs in the back of the tv, vrs straight cable. The signal gets split into its own wires vrs just a few. This means the signals are much clearer as they have their own bandwidth, not sharing. VGA is a very old system, its been around since the days when those huge CRT monitors were new. Consequently, its a very basic red/green/blue color scheme that'll handle 24 distinct colors. It's an analog signal. (think that's right). Dvi, hdmi and display port are all digital. They'll carry over a million distinct colors.
Honestly, at this point, it'd be worth it to spend the $20, buy a cheap video card that has 2x dvi connectors and use a dvi to VGA adapter plug on the touchscreen. You'll get a much better picture and since the gpu is dedicated, you'll not be using the cpu to power the picture. That'll keep everything running much easier, less heat, better quality picture, and better performance.