Have you updated Vista with all the current updates? Have you gone thru the Microsoft KB artilces and gotten the updates available there that apply to the Hibernation issues? I know of at least 2 that were not sent thru WU.
Check those 2 areas for updates related to Vista Hibernation. Sadly not all updates were pushed out thru Windows Update. Most of these are resolved with SP1 which is slated for March release.