Enable/Disable Database Vault

Database Vault - Used to protect objects in a database from access by other users (even by privileged users like sys).
Disable Database Vault-
cd $ORACLE_HOME/rdbms/lib
make -f ins_rdbms.mk dv_off
relink oracle
Enable Database Vault
cd $ORACLE_HOME/rdbms/lib
make -f ins_rdbms.mk dv_on
relink oracle
Now to Ensure that Database Vault has really been enabled, do this-
sys> select * from v$option where lower(parameter) like '%vault%';
If the output is true then database vault has been enabled successfully.
If false then one has to check why it has not been enabled.

Note:-

In case of a Rac Setup -

srvctl stop database -d db_name

note: do this from all nodes:
cd $ORACLE_HOME/rdbms/lib

make -f ins_rdbms.mk dv_off ioracle