Buttercup looks to be using AES-CBC with PBKDF2 and no authentication, but I only took a very brief look so I may have missed important details. That’s not secure if an attacker can alter the vault file, and PBKDF2 isn’t a great KDF to use. If you use this, you definitely need a 128-bit or higher entropy passphrase (10 Diceware words). You usually want that anyway, but using a weaker string for your master password will be less secure than you expect compared to something using a modern KDF.