You should have the Fn-key to swap screen output between external VGA and laptop display working. It's rare that this key handling is not enabled and functioning. But this is only true when you are logged in, so sometimes one has to login "blindly", wait a moment, then hold down the <Fn> key and press the key to toggle between output screens. On an EeePc it is Fn+F8.
Talking about EeePC:s - On these laptops, when having dual boot Win+Lin the energy saver stuff sometimes get things wrong, especially when changing screen outputs, and dims the LCD display right down to totally black. Can be quite confusing in combination with not knowing if the video gets out on a VGA port, HDMI or the internal LCD. Puts a smile on your face when you press the Fn+F6 a couple of times and your video output shines up on the LCD, just as you wanted.