virtualbox.org

When you close the VirtualBox program, the big .vdi file is read for minutes generating a lot of hard disk activity.

Steps to reproduce this bug:
1. open Virtual Box
2. execute a virtual machine, for example Ubuntu
3. exit from Ubuntu and close Virtual Box
4. open the Resource monitor from the Vista's task manager to see the reading of the whole file ubuntu.vdi

Windows Vista SP1.

I would consider this normal behaviour, because the file needs to be saved. But if you think it's really a bug, you can go to the Bugtracker (separate account needed) and report it as such.
There is one thing you need to note here before creating the report. Note all your VM settings, so we give a possible cause for it without the report. It's possible that your file is just too big and heavily fragmented.