According to the Doki Doki Literature Club wiki, have a nice weekend! is a file which is created in Act 2 during the weekend which the player spends with
reading as follows:
This text is encoded first in Base64 and also by a Vigenère cipher with the key "libitina." When decoded it reveals a short sentence:
What is a man without knowing the rich aroma of the future; the hot, complex balance of the present; and the bittersweet aftertaste of the past?
My question is, how is the Vignere cipher and Base64 decoding applied to get the secret message? I've found multiple references to using a Vigenere cipher and Base64 decoding, but nothing actually explaining how to set up the cipher.