Oracle 12C: connecting within container to another database in Oracle 12C

In my Oracle 12.0.1, I cannot connect to a schema directly without specifying the services name.

alter session set container=PDBVCDB;

SQL> conn vpxd/oracle
ERROR:
ORA-01017: invalid username/password; logon denied

Warning: You are no longer connected to ORACLE.

SQL> conn vpxd/oracle@pdbvcdb
Connected.


It probably trying to find the vpxd schema at the root level.