Help with HDF_VD_WRITE - field names

Hi everybody

I'm trying to write a Vdata to an HDF file, but for some reason, the
only field description IDL accepts is 'PX', like the example in the
IDL user's guide.

The command

HDF_VD_WRITE, Vdat, 'PX', intarr(5)

works fine, but

HDF_VD_WRITE, Vdat, 'RR', intarr(5)

returns

HDF_VD_WRITE: Unable to set the VDATA fieldnames to <STRING   ('RR')>.

I need to set the field name to something that I can understand later,
so PX is not an option... Any ideas on how to sole this? I know I can
write as an SDS, but I need to do as Vdata for compatibility with some
files we process here.

Thanks!