The system is entering Away Mode.
i had hibernation problems for a long time since i installed linux in dual boot on my computer (i suspect windows scandisk for ****ing all up), but i finally found how to make my vista hibernate. However, my event log did not show any bitlocker/disk management problems, it was just saying "The system is entering Away Mode." So for me, the problem was not a system related problem, or a file corruption, vista just refused to enter in sleep mode, even though i had the same symptoms than you, the screen went black, and than 2-3 seconds after it went back to the locked thingy. Actually, the solution was only to, in the advanced power options of your power plan, change to "Allow the computer to sleep" in "Multimedia settings/When sharing media", enable "USB Selective suspend setting" (to prevent wake-ups), and deactivate "Allow hybrid sleep" in "Sleep". If this doesnt work, try to run "CMD" and type "powercfg -hibernate off" and then "powercfg -hibernate on". If all this doesnt work, suspect a hardware or bios related problem or a usb or application preventing vista to go in hibernate procedure, if this is the case, i wish you good luck to find the problem.
Hope this will helps,
Dom