file_id	IN:	File identifier returned by Hopen
	vdata_ref	IN:	Vdata reference number

	Purpose	Sequentially searches through a file for vdatas.
	Return value	Returns the reference number for the next vdata if successful and FAIL (or -1) otherwise.
	Description	VSgetid sequentially searches through a file identified by the parameter file_id and returns the reference number of the next vdata after the vdata that has reference number vdata_ref. This routine is generally used to sequentially search the file for vdatas. Searching past the last vdata in a file will result in an error condition.
		To initiate a search, this routine must be called with the value of vdata_ref equal to -1. Doing so returns the reference number of the first vdata in the file.

	FORTRAN	integer function vsfgid(file_id, vdata_ref)
		integer file_id, vdata_ref