On a few occasions, I've run into weirdness with networking on Vista under parallels. I can get a DHCP lease, but I can't ping anything. Yet, I'm clearly connected.
When I ran ipconfig, I discovered that I had two default gateways showing up, the first being 0.0.0.0. Well, that's not going to work! To force your routing table to clear, you can run the following command as an admin in the command prompt within Vista:
route -f
Now that clears all of the gateway entries. You'll need something to get around the network, so simply running ipconfig /release followed by ipconfig /renew and I was good as new and able to surf the network!