The SensorXmpp and ActuatorXmpp projects in the MIoT repository on GitHub provide the support for XEP-0054 to publish contact information using vCards. If you're interested in learning how to implement your own extensions, search for 0054 in the code to see how to register handlers and send stanzas.
The SensorXmpp project also includes a method of publishing current momentary values using the publish/subscribe pattern provided by the presence stanza. This behavior is commented out in the default version, to avoid spamming the broker. If you want to play with this pattern, check the PublishMomentaryValues() method.