In the book Serious Cryptography
It says for a cipher's permutation to be secure
"Different keys should result in different permutations. Otherwise it becomes easier to decrypt without the key: if different keys result in identical permutations, that means there are fewer distinct keys than distinct permutations, and therefore fewer possibilities to try when decrypting without the key. In the Vigenere cipher, each letter from the key determines a substitution; there are 26 distinct letters, and as many distinct permutations."
Cam someone explain this please?