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

What about lowercase?

import tkinter as tk
from tkinter import ttk
from tkinter import *

root = Tk()

root.geometry('826x567')
root.configure(background='#000000')
root.title('Message Encryption and Decryption')

lblInfo=Label(root, text=':::::SECRET MESSAGING:::::::: \n :::Vigenère cipher:::', bg='black', font=('helvetica', 22, 'normal'),fg='purple').place(x=264, y=36)

rand = StringVar()
Msg = StringVar()
key = StringVar()
mode = StringVar()
Result = StringVar()

def qExit():
root.destroy()

def Reset():
rand.set("")
Msg.set("")
key.set("")
mode.set("")
Result.set("")

lblReference = Label(root,bg="black", font = ('Century', 11, 'bold'),text = "NAME:::",fg="white", bd = 11, anchor = "w").place(x=98, y=197)
txtReference =Entry(root, font = ('Century', 11, 'bold'),textvariable = rand, bd = 10, insertwidth = 4,bg = "white", justify = 'right').place(x=343, y=198)
lblMsg =Label(root,bg="black", font = ('Century', 11, 'bold'),text = "MESSAGE:::",fg="white", bd = 11, anchor = "w").place(x=98, y=241)
txtMsg =Entry(root, font = ('Century', 11, 'bold'),textvariable = Msg, bd = 10, insertwidth = 4,bg = "white", justify = 'right').place(x=343, y=241)
lblkey =Label(root,bg="black", font = ('Century', 11, 'bold'),text = "KEY:::",fg="white", bd = 11, anchor = "w").place(x=98, y=274)
txtkey =Entry(root, font = ('Century', 11, 'bold'),textvariable = key,fg="black", bd = 10, insertwidth = 4,bg = "white", justify = 'right').place(x=343, y=275)
lblmode =Label(root,bg="black", font = ('Century', 11, 'bold'),text = "Mode==",fg="white", bd = 11, anchor = "w").place(x=98, y=316)
txtmode =Entry(root, font = ('Century', 11, 'bold'),textvariable = mode, bd = 10, insertwidth = 4,bg = "white", justify = 'right').place(x=343, y=316)
lblService = Label(root,bg="black", font = ('Century', 11, 'bold'),text = "The Result--",fg="white", bd = 11, anchor = "w").place(x=98, y=354)
txtService = Entry(root, font = ('Century', 11, 'bold'),textvariable = Result, bd = 10, insertwidth = 4,bg = "white", justify = 'right').place(x=343, y=354)

import base64

def encode(key, clear):
enc = []

for i in range(len(clear)): 
    key_c = key[i % len(key)] 
    enc_c = chr((ord(clear[i]) +
                 ord(key_c)) % 256) 
                   
    enc.append(enc_c) 
      
return base64.urlsafe_b64encode("".join(enc).encode()).decode() 

def decode(key, enc):
dec = []

enc = base64.urlsafe_b64decode(enc).decode() 
for i in range(len(enc)): 
    key_c = key[i % len(key)] 
    dec_c = chr((256 + ord(enc[i]) -
                       ord(key_c)) % 256) 
                         
    dec.append(dec_c) 
return "".join(dec) 

def Ref():
print("Message= ", (Msg.get()))

clear = Msg.get() 
k = key.get() 
m = mode.get() 

if (m == 'e'): 
    Result.set(encode(k, clear)) 
else: 
    Result.set(decode(k, clear))

btnTotal =Button(root, text='Show Message', bg='#7FFF00', font=('verdana', 12, 'normal'), command=Ref).place(x=112, y=444)
btnReset =Button(root, text='Reset', bg='#FF3030', font=('verdana', 12, 'normal'), command=Reset).place(x=250, y=444)
btnExit =Button(root, text='Exit', bg='#A020F0', font=('verdana', 12, 'normal'), command=qExit).place(x=315, y=444)

root.mainloop()

Vigenere Cipher for any Unicode text

[ ! ] text and key have to be unicode encoding

def vigenere(text: str, key: str, encrypt=True):

    result = ''

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

        if encrypt:
            value = (letter_n + key_n) % 1114112
        else:
            value = (letter_n - key_n) % 1114112

        result += chr(value)

    return result
    

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


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

Example

>>> vigenere_encrypt(u"Hello world!", u"Leet1337:)")
'\x94ÊÑà\xa0Sª¦¬\x95°\x86'

>>> vigenere_decrypt(u"\x94ÊÑà\xa0Sª¦¬\x95°\x86", u"Leet1337:)")
'Hello world!'

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!! :)