I don't know what "viewport" does, but ...
"virtual" is a representation of your "dual monitors".
Your settings on your might be 1024x768, but
in a dual set-up, your "virtual" size is 2048x768, (the two monitors).
Glad to be of assistance.
You should try "glxgears" just to see if you have 3D support,
and to get some idea of what your video card is producing.
Cheers ... take care!