Vista has no ne2000 driver, and the rtl8139 driver does not work with the qemu emulated device.
To get Vista to network correctly, specify the ne2k_pci device on the qemu command line:
qemu -net nic,model=ne2k_pci
and use the Windows 2000 driver disk, available here.
Note: I found that the rtl8139 driver worked with kvm and Vista just fine. I also found the download link above to be broken. So you might also try:
qemu -net nic,model=rtl8139
Note: that's because the emulated rtl8139 was fixed with the last QEMU CVS merge. Vista guests should use rtl8139, as Vista's rtl8139 driver now works.