If I use the encrypt/decrypt functions in vi (or a vi clone) how secure is the resulting encrypted data likely to be?

I know that this encryption is not inherently very secure as it's just a "one-rotor machine designed along the lines of the German Enigma, but with a 256-element rotor".

However given that the encrypted file has no clue to indicate that it's encrypted or the algorithm used to encrypt it I think it would actually be rather difficult to decrypt.

The potential code breaker has to:- Identify the file as being the one with the required data. Guess the algorithm used to encrypt it. Know the contents (or part of the contents).

Am I right in my thinking or does a file encrypted in this way show any clues to how it was created?