You can read about the Vigenere Cipher here: https://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher
The core principle to cracking the cipher is exploiting the fact that the key is repeated for long lengths of text and then performing frequency analysis on each subset of characters separately.
Key length: Longer keys (6-12 characters) provide better security
Key randomness: Avoid dictionary words or predictable patterns
Ciphertext length: At least 200+ characters recommended for accurate cryptanalysis
Text quality: Works with standard alphabetic English text