The standard /vault/core/model/db/mysql/vw.php line 2 doesn't send output. It is a comment block. Since this error is claiming that line is sending output, you may have incorrectly edited the file, or you may have used a code editor that inserts byte-order-marks (BOM). Please check that you did not add any mistakes around line 2. If you can't see any problems near the beginning of the file, such as an extra character or a space before the opening <?php tag, you should use an editor that doesn't add these marks.