Grib file coded with second order packing (maybe only by legacy gribex code) cannot be processed by vg6d_transform, the following error appears when coding output:
GRIB_API ERROR : unable to get codedValues as double array (Key/value not found)
GRIB_API ERROR : unable to get bitsPerValue as long array (Key/value not found)
GRIB_API ERROR : grib_set: values Key/value not found
A palliative is to repack the data with the command grib_set -r -s packingType=grid_simple in out.