Have you been in your power settings and disabled all power saving settings? Try that and see what happens. If that doesnt sort it then try everything ive suggested below. Do you leave the AC power connected to the laptop?If none of this works then it might just be a bug in Vista.
Did you ever check your network card properties like i suggested? If you goto device manager and check the properties of the card. Look in the advanced tab settings and the power management settings there. Look for something like "Wake from Shutdown" mode and turn it on.
Like i said earlier, the system BIOS must be capable of responding to Plug and Play and power management events. By definition, any system with an ACPI BIOS includes this capability. Non-ACPI computers with a Plug and Play BIOS are capable of performing asubset of Plug and Play functions but will not be as capable as ACPI computers. So make sure you definately have the latest BIOS.
Make sure you have all the latest drivers and that they are compatable with Vista. The device driver must be capable of interacting with the operating system and responding to device notification and power management events. A Plug and Play driver can load automatically when Windows detects that a device has been plugged in, and it can suspend and resume properly along with the system. If this driver isnt 100% compatable, then this could be causing your problems.
Ive been doing a bit more digging around aswell. Your're not the only one having this problem. This wont solve the problem but it might help till we can work out a proper solution (or MS get their fingers out their arse and sort this issue out). Theres a program called network magic and some people are having success with this, you can get it here www.networkmagic.com
basically all you do when you lose the connection is click on the fix network button and it restores your connection, not a fix, but it might save you having to reboot all the time.
Right, well thats me out of ideas for now, let me know how you get on with all that, and if it doesnt sort it il try to think of some more stuff to try.