I have a binary file encrypted via Vigenere cipher. I need to carry out a known plaintext attack. I know that the encrypted plaintext starts with phrase Attack at Dawn. 
How exactly would I go about doing it. 
Do I keep the encrypted data in binary or do I somehow convert it to string?
Also, my idea is to brute force all possible keywords up to length 14(len(Attack at Dawn)) but that seems like it might take a while. Is there a more optimal solution?
You want to build a plain text attack to get the key. The first letter of the key can be found by the amount of shifting between the initial A of Attack and the first letter of the encrypted text. Then you will get the second letter of the key from the shift amount of the second letter of the message.
You can then iterate on all the letters of the initial part that you know.
But without more info I cannot say whether upper and lower case letters matter, not if the spaces are encrypted or left apart...
Vigenere decryption of binary file without knowing the key in Python3