i added virtuemart_order_id
result on my csv is :
"invoice_number","virtuemart_order_id"
"1601045IVD02","5782"
"1601045IVD02","5782"
"1601045IVD02","5782"
"1601045IVD02","5782"
"1601045IVD02","5782"
"1601045IVD02","5782"
"1601045YJF01","5783"
"1601045YJF01","5783"
"1601045YJF01","5783"
"1601045YJF01","5783"
"1601045YJF01","5783"
"1601045YJF01","5783"
"1601045YJF01","5783"
"160104JBEH04","5779"
"160104JBEH04","5779"
"160104KPIG03","5780"
template :
<?xml version="1.0"?>
<csvi>
  <template>
    <settings>
      <action><![CDATA[export]]></action>
      <component><![CDATA[com_virtuemart]]></component>
      <operation><![CDATA[order]]></operation>
      <exportto>
        <option><![CDATA[todownload]]></option>
      </exportto>
      <localpath><![CDATA[/home/mywebsite/public_html]]></localpath>
      <ftphost><![CDATA[]]></ftphost>
      <ftpport><![CDATA[]]></ftpport>
      <ftpusername><![CDATA[]]></ftpusername>
      <ftppass><![CDATA[]]></ftppass>
      <ftproot><![CDATA[]]></ftproot>
      <ftpfile><![CDATA[]]></ftpfile>
      <export_email_addresses><![CDATA[]]></export_email_addresses>
      <export_email_addresses_cc><![CDATA[]]></export_email_addresses_cc>
      <export_email_addresses_bcc><![CDATA[]]></export_email_addresses_bcc>
      <export_email_subject><![CDATA[]]></export_email_subject>
      <export_email_body><![CDATA[]]></export_email_body>
      <export_filename><![CDATA[testdenis.csv]]></export_filename>
      <export_file><![CDATA[csv]]></export_file>
      <field_delimiter><![CDATA[,]]></field_delimiter>
      <text_enclosure><![CDATA["]]></text_enclosure>
      <include_column_headers><![CDATA[1]]></include_column_headers>
      <signature><![CDATA[0]]></signature>
      <publish_state><![CDATA[1]]></publish_state>
      <recordstart><![CDATA[]]></recordstart>
      <recordend><![CDATA[]]></recordend>
      <export_date_format><![CDATA[d/m/Y]]></export_date_format>
      <export_price_format_decimal><![CDATA[2]]></export_price_format_decimal>
      <export_price_format_decsep><![CDATA[,]]></export_price_format_decsep>
      <export_price_format_thousep><![CDATA[]]></export_price_format_thousep>
      <header><![CDATA[]]></header>
      <body><![CDATA[]]></body>
      <footer><![CDATA[]]></footer>
      <language><![CDATA[fr-FR]]></language>
      <splitorderline><![CDATA[1]]></splitorderline>
      <ordernostart><![CDATA[5778]]></ordernostart>
      <ordernoend><![CDATA[]]></ordernoend>
      <orderlist><![CDATA[]]></orderlist>
      <orderdaterange><![CDATA[]]></orderdaterange>
      <orderdatestart><![CDATA[]]></orderdatestart>
      <orderdateend><![CDATA[]]></orderdateend>
      <ordermdatestart><![CDATA[]]></ordermdatestart>
      <ordermdateend><![CDATA[]]></ordermdateend>
      <orderstatus>
        <option><![CDATA[S]]></option>
      </orderstatus>
      <orderpayment>
        <option><![CDATA[]]></option>
      </orderpayment>
      <order_address><![CDATA[BT]]></order_address>
      <ordermanufacturer>
        <option><![CDATA[none]]></option>
      </ordermanufacturer>
      <ordercurrency>
        <option><![CDATA[47]]></option>
      </ordercurrency>
      <orderpricestart><![CDATA[]]></orderpricestart>
      <orderpriceend><![CDATA[]]></orderpriceend>
      <orderuser>
        <option><![CDATA[]]></option>
      </orderuser>
      <orderproduct>
        <option><![CDATA[]]></option>
      </orderproduct>
      <groupbyfields><![CDATA[{"name":["virtuemart_order_id","invoice_number"]}]]></groupbyfields>
      <sortfields><![CDATA[]]></sortfields>
      <use_system_limits><![CDATA[0]]></use_system_limits>
      <max_execution_time><![CDATA[]]></max_execution_time>
      <memory_limit><![CDATA[]]></memory_limit>
    </settings>
    <template_name><![CDATA[testdenis]]></template_name>
    <advanced><![CDATA[0]]></advanced>
    <action><![CDATA[export]]></action>
    <frontend><![CDATA[0]]></frontend>
    <secret><![CDATA[]]></secret>
    <log><![CDATA[0]]></log>
    <lastrun><![CDATA[2016-02-15 16:32:33]]></lastrun>
    <enabled><![CDATA[1]]></enabled>
    <ordering><![CDATA[0]]></ordering>
    <fields>
      <field>
        <field_name><![CDATA[invoice_number]]></field_name>
        <xml_node><![CDATA[]]></xml_node>
        <column_header><![CDATA[]]></column_header>
        <default_value><![CDATA[]]></default_value>
        <enabled><![CDATA[1]]></enabled>
        <sort><![CDATA[0]]></sort>
        <cdata><![CDATA[1]]></cdata>
        <ordering><![CDATA[2]]></ordering>
      </field>
      <field>
        <field_name><![CDATA[virtuemart_order_id]]></field_name>
        <xml_node><![CDATA[]]></xml_node>
        <column_header><![CDATA[]]></column_header>
        <default_value><![CDATA[]]></default_value>
        <enabled><![CDATA[1]]></enabled>
        <sort><![CDATA[0]]></sort>
        <cdata><![CDATA[1]]></cdata>
        <ordering><![CDATA[1]]></ordering>
      </field>
    </fields>
  </template>
</csvi>