Rules for using a Vernam Cipher key:
1. Must be random.
2. Must not be reused.
3. Must not be compromised (lost outside of cryptanalysis).
4. Must be as long as the ciphertext.
1. The ciphertext is captured complete
2. The language of the message is known (which reveals frequency of its digraphs and trigraphs and grammar)
3. The message is not padded and that is known
4. The format of the message is known (headers, Russian Copulation, etc.)
5. The plaintext injection is certain to have occurred repeatedly over many messages
6. The amount of presumed key code is large
None of these factors will help break the key as long as the key is truly random.
But if the key is weak such as being a passage from a book (not a one-time-pad), then the weak key can become readily apparent, and any of the factors listed above will just accelerate that exposure.