If a process has an open file on the drive, you can't unmount it normally. You have to close the process first (or kill it). Now vlh-umount has options for this:
vlh-umount = normal unmount
vlh-umount -s = unmount with sighup
vlh-umount -k = unmount with sigkill
sighup signals the process to shut itself down, which is better than kill, but won't work if the process is hang up.
The vl-hot desktop icon on KDE has similar unmount options on the context menu, but this was not yet possible to do on XFCE.