This tool can help you encrypt, decrypt, analyze, and crack Vigenère Ciphers. Feel free to use it to analyze the ciphertext from the puzzles each week to discover the key, then use that information to decrypt the puzzle, and encrypt a reply to the @CipherMonkeyTwitter account each week.
To help you get started and a good feel for the tool, here is some Vigenère ciphertext where the key is completely unkown. If you've analyzed everything correctly, you should be able to discover the key, decrypt it, and get to the same plainext as provided:
Ciphertext: KH IU AJS SWWTSMG HTH OH AJS TGHEVET AQ OWCRGB JQF KB CTLCHIXL GLPTLUGIQU CBD MUQKLGKIS. Plaintext: IT IS THE SUPREME ART OF THE TEACHER TO AWAKEN JOY IN CREATIVE EXPRESSION AND KNOWLEDGE.