You can't use LIKE with ejb-ql. Its the law according to O'Reilly's EJB book and I am assuming that the spec agrees with that notion!
To overcome the situation, duplicate your in the jbosscmp-jdbc.xml and replace ejb-ql with jboss-ql and you'd be finer than a girl with shaved legs.
I modified the below for you. Also make sure that when you send the your param in, that it has your appended '%' to it.
<![CDATA[SELECT OBJECT(i) FROM T_Item i WHERE i.vendor.name LIKE ?1]]>