As others have said, USB is not an option. To put it in numbers, USB can carry (theoretically, not realistcally) 480 Mbps. In comparison, the bus used by video cards today, PCIe 16x can carry 64,000 Mbps.

Your best bet would be to get a docking station that has support for adding in a PCIe card. If there is no such docking station but the laptop has an ExpressCard/54 slot (most newer laptops do), you can maybe get a device that gives you an external case for a video card. The Asus XG station was supposed to be released in May, 2007, but I didn't see any recent information about it and no stores selling it. Plus it costs $600, while docking stations cost $300-$400.

Also, even if a docking station has both DVI and VGA ports, check to make sure it can actually run both at the same time (this will almost definitely require shutting off the LCD screen).