In includes/function_login.php around line 449 (the last line in function do_login_redirect()) is this:
Code:
print_standard_redirect(array('redirect_login',$vbulletin->userinfo['username']), true, true, $vbulletin->userinfo['languageid']);
If you edit that file and change the 3rd parameter (the second 'true' which is marked in red) to false, it should eliminate that message. Offhand I don't see any way to do it in a plugin.