Ethernet devices are common enough that you shouldn't need to download any additional drivers for ethernet connectivity when using Vista, as the drivers should be included with the OS. Although if it is some freaky laptop made by the fine people at
Skoda, you might be outta luck.
What model of laptop is it?
What does device manager list the ethernet device as? Broadcom?
Is it enabled and functioning properly, or does it have a yellow exclamation mark beside it?
Are you sure it worked before you reinstalled Vista, or could it have fried the device via power surge or something? If you use wireless mostly, you might not have noticed?
At any rate, you could try deleting the drivers through device manage, and rebooting the system. Vista should reinstall them automaticly and you should be good to go.