Vista stops recognizing external hard drive

This doesn't sound like an OS issue. This appears to be more an issue of the connection to the hard drive. The USB cable likely isn't the culprit, nor the laptop USB inputs. If the drive was ever dropped or the cable gets removed and reattached often to the actual enclosure, you might be looking at a bad port on the enclosure. This isnt a lost cause. You can always buy a hard drive enclosure from newegg and move the drive from the IOMEGA to the new enclosure. You can get a new enclosure for $15. Just because the USB port is bad, it doesnt mean the data is gone and the hard drive cant be used.

seems you have tride there is maybe only one suggeston
you may have dry joints/ inside your external via- your usb which needs soldering you shold get someone to check for you hope this helps