Wolfram Function Repository
Instantuse addon functions for the Wolfram Language
Function Repository Resource:
Perform Vigenère's substitution cipher on a string
ResourceFunction["VigenereCipher"][string,key] enciphers string using the key key. 

ResourceFunction["VigenereCipher"][string,{key_{1},key_{2},…}] repeatedly enciphers string using first key_{1}, then key_{2}, etc. 

ResourceFunction["VigenereCipher"][{s_{1},s_{2},…},key] enciphers the strings s_{1},s_{2}, etc. using the key key. 

ResourceFunction["VigenereCipher"][key] represents an operator form of ResourceFunction["VigenereCipher"] that can be applied to an expression. 
Encipher a message using the key "LEMON":
In[1]:= 

Out[1]= 

Encipher with multiple keys:
In[2]:= 

Out[2]= 

Encipher a message twice, first using "GO" and then "CAT":
In[3]:= 

Out[3]= 

Encipher multiple strings:
In[4]:= 

Out[4]= 

Create an operator that enciphers strings with the key "sparks":
In[5]:= 

Out[5]= 

Apply the operator to a string:
In[6]:= 

Out[6]= 

If a single character is used, the Vigenère cipher is identical to the Caesar cipher:
In[7]:= 

Out[7]= 

When enciphering with multiple keys, the keys can first be enciphered with each other to encipher the message with a single key. The length of the replacing key has to be the LCM of the lengths of each of the keys:
In[8]:= 

Out[8]= 

This indeed matches with what you expect:
In[9]:= 

Out[9]= 

The case of the original text is retained; the case of the key does not matter:
In[10]:= 

Out[10]= 

In[11]:= 

Out[11]= 

Spaces and the letters a and A in the key will not alter the original message:
In[12]:= 

Out[12]= 

The key should have valid letters and valid length:
In[13]:= 

Out[13]= 

In[14]:= 

Out[14]= 

In[15]:= 

Out[15]= 

Two keys can negate each other:
In[16]:= 

Out[16]= 

Enciphering the keys with each other gives a string with only As:
In[17]:= 

Out[17]= 

Spaces, numbers and letters outside the regular English alphabet are not transformed:
In[18]:= 

Out[18]= 

Use RemoveDiacritics, StringDelete, IntegerName and ToLowerCase to transform the message:
In[19]:= 

Out[19]= 

Enciphering the transformed message results in a muchhardertocrack cipher:
In[20]:= 

Out[20]= 

This work is licensed under a Creative Commons Attribution 4.0 International License