In order to remove a virtual domain you have to send VHOSTS_REMOVE command to the server with the domain name.

The domain remove command is sent by the client:

<iq type="set"
    to="vhost-man@existing.domain.com"
    id="aacba">
  <command xmlns="http://jabber.org/protocol/commands"
           node="VHOSTS_REMOVE">
    <x xmlns="jabber:x:data" type="submit">
      <field type="text-single"
             var="VHost">
        <value>virt-nn.domain.com</value>
      </field>
    </x>
  </command>
</iq>

Upon successful completion of the command the server sends a response back to the client with information of the existing number of virtual hosts on the server:

<iq from="vhost-man@existing.domain.com"
    type="result"
    to="cmd-sender-admin@existing.domain.com"
    id="aacba">
  <command xmlns="http://jabber.org/protocol/commands"
           status="completed"
           node="VHOSTS_REMOVE">
    <x xmlns="jabber:x:data" type="result">
      <field type="fixed" var="Note">
        <value>Current number of VHosts: 124</value>
      </field>
    </x>
  </command>
</iq>