Vigenère cipher: Security when key length and plaintext length are the same I have read the Vigenère cipher is secure as long as the key length is the same as the length of the data to be ciphered.

Is this true when the same key is used multiple times?
In such cases, after how many uses would the key be considered insecure?
Forming my comment into an answer:

If the key has the same length as the message and is used only once, it is basically a One-Time-Pad. This means, that in theory you can match any ciphertext to any plaintext with $a key$. If this key has to match certain criteria (e.g. be a word of a certain language), the information theoretic aspect will be lost. It depends on the actual keyspace if this is a problem.

However, the re-usage is more tricky, because all security is lost in Vigenere once a key is reused. If you have two ciphertexts created by the same key, you can just combine them and have the key eliminated. Then you are left with the combination of the two plaintexts, which is easy to deal with by using frequency analysis.