What is the difference between a Vigenère cipher and a one time pad cipher?
The Vigenère cipher is essentially a series of interwoven Caesar ciphers, in which the size of the shift is different for each character and is defined by the keyword.

The one-time pad (OTP) is a very similar technique in which the only difference is that key is generally:


truly random,
at least as long as the message,
and never reused (thus the name),


making it provably confidential if the key is kept secret.

In this way, both ciphers make use of modular addition, which is still in use in most modern digital cryptography. But, in the case of the OTP, the length of the key guarantees that the ciphertext is not vulnerable due to repetition in the way that the Vigenère cipher is.