Certain capture devices for VGA/DVI are not able to pickup the correct resolution if does not yet have a signal (no VGA source attached yet). So when you start the encoder application it will show a black screen which has the default device resolution. If you lateron attach the VGA source, it will often show up but it will still be in the initial resolution. The resulting recording will have slide that are probably not be very readable.
For the VGA2USB device often a trick is applied to show a default image at a fixed resolution so the device will be in the right resolution even if no signal is available. This trick is described in another article in the FAQ.
To prevent this situation from happening, you should first attach the VGA source before starting up the encoder application.