I'm struggling to find out whether or not I should set up SFTP since I already connect to the internet using a secure VPN.
Is the security using FTP through a VPN (using OpenVPN, more specifically https://mullvad.net) as secure as SFTP?
SFTP (as in, file transfer over an SSH tunnel) provides end-to-end encryption, plus verification of the server's identity. FTP over VPN provides encryption from your computer to the VPN endpoint, and no verification of the server's identity.
If the VPN endpoint is the same computer as the FTP server, then FTP over VPN is as secure as SFTP. In any other circumstance, SFTP is more secure.