Simple Vigenere Cipher written in Python 3.5.

vigenere.py

def encrypt(plaintext, key):
    key_length = len(key)
    key_as_int = [ord(i) for i in key]
    plaintext_int = [ord(i) for i in plaintext]
    ciphertext = ''
    for i in range(len(plaintext_int)):
        value = (plaintext_int[i] + key_as_int[i % key_length]) % 26
        ciphertext += chr(value + 65)
    return ciphertext


def decrypt(ciphertext, key):
    key_length = len(key)
    key_as_int = [ord(i) for i in key]
    ciphertext_int = [ord(i) for i in ciphertext]
    plaintext = ''
    for i in range(len(ciphertext_int)):
        value = (ciphertext_int[i] - key_as_int[i % key_length]) % 26
        plaintext += chr(value + 65)
    return plaintext

Vigenere Cipher for specific alphabet

def vigenere(
        text: str, 
        key: str, 
        alphabet='0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz',
        encrypt=True
):

    result = ''

    for i in range(len(text)):
        letter_n = alphabet.index(text[i])
        key_n = alphabet.index(key[i % len(key)])

        if encrypt:
            value = (letter_n + key_n) % len(alphabet)
        else:
            value = (letter_n - key_n) % len(alphabet)

        result += alphabet[value]

    return result


def vigenere_encrypt(text, key):
    return vigenere(text=text, key=key, encrypt=True)


def vigenere_decrypt(text, key):
    return vigenere(text=text, key=key, encrypt=False)

Example

vigenere_encrypt('Python', 'secretWORD')
'HcVYSg'

vigenere_decrypt('HcVYSg', 'secretWORD')
'Python'

---

P.S. Please note that the alphabet from the example is missing a space, so trying to encrypt several words will throw an exception!

vigenere_encrypt('Hello world', 'SecRet')
Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "<input>", line 34, in vigenere_encrypt
  File "<input>", line 16, in vigenere
ValueError: substring not found

For fix it, just add space into the alphabet

...
alphabet='0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz ', # <--- HERE
...

bravo Raihan!! :)