typedef struct _VPB
{
     SHORT Type;
     SHORT Size;
     WORD Flags;
     WORD VolumeLabelLength;
     PDEVICE_OBJECT DeviceObject;
     PDEVICE_OBJECT RealDevice;
     ULONG SerialNumber;
     ULONG ReferenceCount;
     WCHAR VolumeLabel[32];
} VPB, *PVPB;

Windows Vista Kernel Structures