Hi Vinh,
I guess you mean the mini din 8 connector in the waysmall or tweener.
Its just a different connector, RS232 can be configured in fact with any connector, provided the voltages and needs of the rs232 standard are followed. The connector you name vga is the DB-9, and is commonly used in peecees for the COM port, you also have DB-25, but less used. You can get a usb<-->rs232 adapter for your lappie, in fact is useful for uploading root images and common debugging.

Using usbnet, well, i dont know, but if you wait for gumstix to boot and configured the bridge in windows or linux, after some minutes plug the cable of usb and try to "ssh -l root <IP address of the gumstix>", that "should" work, but cant tell