here is the trick, put /var/www/vhosts/mywebsite.com path before every path:
mysqldump --user=admin --password=youradminpassword --all-databases > /var/www/vhosts/mywebsite.com/backup/mysqlall-`/bin/date +\%Y\%m\%d`.sql
tar -cvzf /backup/mywebsite.com-backup-`/bin/date +\%Y\%m\%d`.tar...