It won't work. Do you have securityAssist? You could add a rule pretty easily.
Validation toolkit can't be used to show any messages if the form submits successfully.
Instead you should add a url parameter to a redirect after a successful form submit. Then use that url parameter to show the success message. If you aren't using a redirect you can use :
<?php if (isset($_POST)) { ?>
around the error message. Since the only way a post would go through is after a successful validation. SecurityAssist comes with a show conditional region you can use to create the if statement for you, so you wouldn't have to write the code, that is why I asked.