Also I'm unsure what you are trying to do there:

x=1
x=`nzsql -vON_ERROR_STOP=1 -c "update ADMIN.OFFER_TBL_BULK_LOAD set CFLSTPRN_SEGMENT_DESC = '${vals[1]}' where CFLSTPRN_MAILED_SEG_ID = '${vals[2]}' and CFLSTPRN_MAILED_SEG_SUFF = '${vals[3]}';"`
exit $x

x=`command` is going to trap the output of the nzsql command, not thre return code.  I think what you intended is this:

nzsql -vON_ERROR_STOP=1 -c "update ADMIN.OFFER_TBL_BULK_LOAD set CFLSTPRN_SEGMENT_DESC = '${vals[1]}' where CFLSTPRN_MAILED_SEG_ID = '${vals[2]}' and CFLSTPRN_MAILED_SEG_SUFF = '${vals[3]}'"

exit $?

Rather than using -c "SQL statement" I prefer to use a HERE document, but either format works:

nzsql -vON_ERROR_STOP=1 <<-END

     update OFFER_TBL_BULK_LOAD

        set CFLSTPRN_SEGMENT_DESC = '${vals[1]}'

      where CFLSTPRN_MAILED_SEG_ID = '${vals[2]}'

        and CFLSTPRN_MAILED_SEG_SUFF = '${vals[3]}'

END

exit $?