If when using a Vignere cipher I replace the key word often with a word earlier in the ciphered plain text would this be stronger due to not being able to do analysis with a repeated key or would it be fundamentally weaker due to sending the key for the next message for everyone to see? How long do the messages need to be in terms of key length to make this method stronger?
For example: The key word orange has been shared with 2 parties. The message "my password is bob and the next key word will be science" is encrypted using the word orange and sent to the second party. They then send me back a message using the key word science to encode their message as well as sending me a new key word.