input: Write a function to encrypt a plaintext using the Vigenère cipher.
  assert encrypt_vigenere("HELLO", "KEY") == "RIJVS"
  assert encrypt_vigenere("ATTACKATDAWN", "LEMON") == "LXFOPVEFRNHR"
  assert encrypt_vigenere("VIGENERECIPHER", "ABC") == "VIGENERECIPHER"
output: def encrypt_vigenere(plaintext, key):
      def shift_char(c, k):
          return chr(((ord(c) - ord('A') + ord(k) - ord('A')) % 26) + ord('A'))
      
      key = (key * (len(plaintext) // len(key) + 1))[:len(plaintext)]
      ciphertext = ''.join(shift_char(p, k) for p, k in zip(plaintext, key))
      
      return ciphertext