This is actually the forum for Audit Vault not Database Vault. But to answer your question, If you want to restrict User A from accessing any tables in Schema B other than the EMP table, then you can just use normal object level privileges. i.e.
GRANT SELECT ON B.EMP TO A;
If User A has some ANY privileges that would allow them to access other tables in Schema B, such as SELECT ANY TABLE, then you should put Schema B in a Realm to protect it from the ANY privileged users. Any other users that should still be able to make use of the ANY privileges can then be added as Realm Participants.