Hi,
I'm no jboss expert and using jboss 5.0.1 GA, not sure if this configuration works in 4.0, but I think, you are referring to wrong xml file, cause jboss creates <jndi-name>-ds.xml , so
you should have a file named vgi-ds.xml.
I may be wrong here.
Also, did you restart the server, after making the changes in the xml ?