If I use a Canon or Nikon lens on the GH2 does the auto focus function become disabled? What about if the lens doesn't have an aperture ring? Will that be possible to control in camera?

Auto-focus will be disabled. Without an aperature ring on the lens you'll have to buy an adaptor with that feature.

You will be able to control the shutter speed and ISO however.

Lots of lenses however have manual apertures built in. For me, a manual vintage lens is far easier to shoot with than these auto everything do nothing quite right lenses.

I have the Voigtlander 25mm already, but that lens is purely manual. I was just wondering if I were to get a lens that wasn't purely manual. In that case I'll definitely need a lens with an aperture ring. I don't care about autofocus as much.

Any non-native MFT lens e.g.Canon or Nikon will only be a manual lens with current adaptors. So no AF & no IS even if the lens does sport these features. When the Birger adaptor is available it should allow Canon EF lenses (possibly only 'L' ones as the Birger description is ambiguous) to be used with AF & IS.