I suggest read the free EJB 2.0 (PDF) book available at the serverside.com or get a copy of the O'Reilly EJB book.
EJB-QL is based on the abstract schema of the entity beans. The from clause uses the abstract-schema-name declared for the entity in the ejb-jar.xml file (not the table name), and the where clause uses the cmp-field name. Your query should look something like this:
FROM vector_user a
WHERE a.username = ?1 AND a.password = ?2
Notice I added the alias declaration a, and used that alias in the where clause. This query assumes that the abstract-schema-name vector_user is valid and it has username and password cmp fields.