Within a closed computer network, I am ciphering some plaintext data as an added security measure. This is below several other layers of protection. For various technical reasons, I am restricted to plain-text, UTF-8 data. To that end, I have been using a Vigenère cipher with pre-shared key database. The keys range from 30 to 100 characters, and are not dictionary words.
My question is regarding the Vigenère cipher: It is my understanding that the security of this cipher is directly related to the length and security of the keys. Long and tightly secured keys bring this cipher on par with many more complex techniques. Is that true, or is this a "kiddie" cipher that could be cracked while you sleep?
Edit It may bear mentioning, the text that is being ciphered is JSON encoded data arrays, which is human readable but not "natural language" -- it contains many symbols interspersed with the actual data.
EDIT 2 Plaintext length varies unpredictably, from 100 characters to 5,000 plus. The data is being transported via SSL, so this is, as I mentioned, intended as yet another added layer, not the whole and sum of security used.