vdata_id	IN:	Vdata identifier returned by VSattach
	filename	IN:	Name of the external file
	offset	IN:	Offset, in bytes, of the location in the external file the new data is to be written

	Purpose	Stores vdata information in an external file.
	Return value	Returns SUCCEED (or 0) if successful and FAIL (or -1) otherwise.
	Description	VSsetexternalfile writes data in the vdata identified by the parameter vdata_id in the file named filename, at the byte offset specified by the parameter offset.
		Only the data will be stored externally. Attributes and all metadata will remain in the primary HDF file.
		IMPORTANT: The user must ensure that the external files are relocated along with the primary file.
		Read the Reference Manual page on SDsetexternalfile for more information on using the external file feature.

	FORTRAN	integer function vsfsextf(vdata_id, filename, offset)
		integer vdata_id, offset
		character*(*) filename