The disadvantage of using encrypted partitions is that while the partition is mounted, at least root can access the data. To prevent this, vi can be used in encrypted mode.
Use vi -x filename to edit a new file. vi prompts you to set a password, after which it encrypts the content of the file. Whenever you access this file, vi requests the correct password.
For even more security, you can place the encrypted text file in an encrypted partition. This is recommended because the encryption used in vi is not very strong.