vlc are in the Universe component (see the Repositories/Ubuntu community documentation wiki page).
aircrack-ng is not provided in Ubuntu 12.04's official software sources (though it is provided in the official software sources for previous Ubuntu releases, and will be provided again in Ubuntu 12.10).
I recommend fixing whatever problem is preventing
vlc from being installed, before deciding which of the available methods you wish to use to install
aircrack-ng (since, depending on how you install that, the same problem might get in the way if not fixed).
So, to install
vlc, first make sure this component is enabled by going to Edit > Software Sources... in the Software Center's top menu bar, and checking the box labeled Community-maintained free and open-source software (universe) (in the Ubuntu Software tab) if it isn't already checked. Then click Close.
That should fix the problem. Unless it was already checked. In that case, or if you know that you're also unable to install packages from the Main component (which is virtually always enabled, and you can see if it is in the same way), try the fix proposed in r000000000000t's answer.
Once this is fixed and you're able to install
vlc, go ahead and install
aircrack-ng using one of the methods detailed in the answers here: