Check if you VM has snapshot... and if Yes, remove the snapshot and try expand the disk again.
You don't need power off the VM to increase the disk, removing snapshot is enough to be able to increase the vdisk
Must delete the snapshot or consolidate it first before extending the disk size.
Windows Server 2003 need a reboot if System Disk (c:\) is being extended.
Above than 2003 doesnt require a reboot. only extend command is enough to extend the disk . Doesn't matter if disk is os or data.