OK, I got the command v-restore-user to work with the file by giving the path to it:
v-restore-user admin /home/backup/admin.2019-05-08_19-33-45.tar
but then I got this error:
Error: invalid backup format :: /home/backup/admin.2019-05-08_19-33-45.tar
Strange, this is Hestia backup and restore without involing Vesta to Hestia retore. Why invalid backup format? I even tried to remove the _19-33-45 part in the finename, but the same error.