Hi @myvesta, thanks for the report! I've split this post into its own thread, because it looks to me like the Vesta ecosystem doesn't rely on Plesk, but instead has its own ACME client.
In between finalize and downloading the issued certifcate, that client needs to poll the order URL (with exponential backoff, please!) until the "certificate" field appears in the Order object. This will be similar to the polling that already happens for authorizations.
Making that change will allow the Vesta client to work with asynchronous finalization, to be compliant with RFC 8555, and to potentially work with ACME Servers other than Let's Encrypt in the future. Thanks!