One idea would be to set vBulletin Options -> Cookies and HTTP Header Options-> Remove Redirection Message Pages to no and edit the 'reportthanks' phrase to whatever you want it to read. Another idea, if you do not want redirection messages in multiple locations, is to edit the 'reportthanks' phrase to whatever you want it to read, find the following line in the report.php file, and add the bit in blue bold so a redirection message appears upon report:
Code:
eval(print_standard_redirect('redirect_reportthanks', true, true));