Finding Vigenere's keys with part of decrypted text?
The key lengths are important, also if it's the Vigenère cipher is applied twice its the same as once with a different keyword. If the two keys are the same length then the new key is that length, if the two keys are different lengths then the new key is sometimes = k1 length * k2 length. If the key length is longer than the message there isn't much analysis you can do.