https://<firewall>//api/?key=apikey&type=config&action=get&xpath=/config/devices/entry/vsys/entry[@name='vsys1']/address
The response looks similar to the following:
<response status="success" code="19">
<result total-count="1" count="1">
<address admin="name" dirtyId="8" time="2015/10/20 15:32:36"><entry name="testobject"><ip-netmask>192.0.2.2</ip-netmask></entry><entry name="test1"><ip-netmask>192.0.2.12</ip-netmask></entry>
...</address>
</result>
</response>