Hi,
It is completely waste of time to try to fix each strict standards error message. Of course no body likes error, and solving is better than hiding them. But you can't handle them all one by one, most of the time it is the best option to hide them.
Because you are saying you have lots of them in the backend too. I wouldn't suggest you to touch any Core files (including Joomla and Virtumart). Because once you update your Virtuemart or Joomla to any newer version your modification will be retrieved back to stock, and those error message will start being appear again. And solving the Strict standard error won't give you any performance boost at all.
Instead of disable the error reporting from the entire website; you may also include that line below to PHP files (that generates error message) individually;
<?php
error_reporting(0);
?>
So only error reporting will be closed only for that individual PHP file.
Regards,
Support