Gajim MUST ask for the vCard using bare JID even in MUC.
According to XEP-0054: [http://www.xmpp.org/extensions/xep-0054.html#sect-id2251433] client MUST ask for the user's vCard using bare JID (without resource)
Gajim does it in the roster but not in MUC, where it uses the full JID (with resource). This has no problem on ejabberd, but causes problems retrieving vCard on jabber2 server.