I mounted a tape using VM:Tape and specified the DSN, but VM:Tape did not update the standard label HDR1 record with the fileid. Why not?
It is your responsibility to ensure that the tape labels are updated. You can do this easily using the CMS LABELDEF and FILEDEF commands with an OS simulation application. The label will be written when the data set is opened. VM:Tape is concerned only that the volser in the VOL1 label is correct.