As many of us having to less holidays it's not surprising that this bug was not recognised so far
Activating the vacation reply will use the 'default' email address, which is wrong in my case (it was changed inside imap settings and can be seen on the top left of sogo window).
This situation will be typically for a multi mail domain setup (all users are placed in the same domain, but at least not all can use this domain for public mails)
You can manually change this email in the vacation setup and it will work - so it is a harmless bug, but having 50 users going to holiday you can guess how many of them will use the wrong email...
If you are using the wrong, no mail will be sent out!
Would be great if the email address from imap setting could be used here: