Possibly, in the Device Manager settings, it is set to automatically turn the device off to save power. Perhaps Vista is turning the LAN off if there is no activity for a few minutes or so. To check:
- Open the Start Menu
- Type in the Search Box "Device Manager" (without the quotes)
- Click on Device Manager in the results
- Enter your password or confirm in the UAC prompt (if enabled)
- After it loads, click on the arrow next to "Network Adapter"
- Right click on your LAN Adapter
- Click Properties
- Click on the tab "Power Management"
- Uncheck the box "Allow the computer to turn off this device to save power"
See if the problem is fixed. If not, then there may be something wrong with the adapter or you may need to update your driver.