Slant parallelogram means: the pivot pins which allow the derailleur cage plates to move are no longer in a plane that is parallel to, say, the main triangle of the bike.
For example, on a Campy NR (or a Campy Rally, for that matter), if you drew a line along the axis of one of the derailleur cage pivot pins, that line would be parallel to the plane formed by the main triangle of the bike frame.
On a Suntour VGT, this is not the case.
Brad Stockwell
Palo Alto