How do you apply a Vignere cipher to decode the have a nice weekend file?

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
Yuri's corpse,
reading as follows:
G2pilVJccjJiQZ1poiM3iYZhj3I0IRbvj3wxomnoeOatVHUxZ2ozGKJgjXMzj2LgoOitBOM1dSDzHMatdRpmQZpidNehG29mkTxwmDJbGJxsjnVeQT9mTPSwSAOwnuWhSE50ByMpcuJoqGstJOCxqHCtdvG3HJV0TOGuwOIyoOGhwOHgm2GhlZpyISJik3J/
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.
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Who is Libitina DDLC?
Libitina is a alter ego of Yuri. She appers in Alternative act 3 as supernatural being who is angry about Yuri's death, her rebuff by MC and Monikas Manipulation. Therefore she want to meet MC.How long is the weekend in DDLC?
The \u201cweekend\u201d refers to the protagonist staring at the decaying corpse of Yuri as time passes for seemingly three days, followed by nothing but corrupted text.What happens in Act 3 of DDLC?
Game Recap: Doki Doki Literature Club Act 3She confesses that she made Sayori's mental depression more severe, causing her sudden suicide, and maximized Yuri's obsessive tendencies. She furthermore explains that she did so because she thought that it would make the player spend more time with her.Is the portrait of Markov a real book?
Portrait of Markov does not appear to be a real book, but \u201cMarkov\u201d is typically in reference to Andrey Markov, a Russian mathematician. Markov's most well-known contribution to mathematics is the Markov chain, a process for selecting outcomes based on one or more previous events.Vigenere Cipher - Decryption (Unknown Key)
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Images: Sora Shimazaki, Andrea Piacquadio, Askar Abayev, Helena Lopes