Which of these code changes are 'safe'/correct?

[Razasharp]

I am trying to change the page a user is sent to after replying to a thread.

With help from members Merk and Andreas I have located the bit of the newreply.php file which I need to change, in particular this bit:

$vbulletin->url = 'showthread.php?' . $vbulletin->session->vars['sessionurl'] . "p=$newpost[postid]#post$newpost[postid]";

which is half way ish down this bit of code:

PHP Code:

if (strtolower($vbulletin->userinfo['lang_charset']) == 'iso-8859-1')
{
    $vbulletin->userinfo['lang_charset'] = 'windows-1252';
}

$db->close();
@header('Content-Type: text/html' . iif($vbulletin->userinfo['lang_charset'] != '', '; charset=' . $vbulletin->userinfo['lang_charset']));
echo "<!-- postbit ok --><!-- time " . TIMENOW . " -->" . process_replacement_vars($postbits);
// #############################################################################
// #############################################################################
// #############################################################################
        }
        else
        {
            if ($newpost['visible'] OR can_moderate($foruminfo['forumid'], 'canmoderateposts'))
            {
                if ($threadview < $threadinfo['lastpost'])
                {
                    $vbulletin->url = 'showthread.php?' . $vbulletin->session->vars['sessionurl'] . "p=$newpost[postid]&amp;posted=1#post$newpost[postid]";
                }
                else
                {
                    $vbulletin->url = 'showthread.php?' . $vbulletin->session->vars['sessionurl'] . "p=$newpost[postid]#post$newpost[postid]";
                }
                $forceredirect = false;
            }
            else
            {
                $forceredirect = true;
                $vbulletin->url = 'forumdisplay.php?' . $vbulletin->session->vars['sessionurl'] . "f=$foruminfo[forumid]";
            }

            ($hook = vBulletinHook::fetch_hook('newreply_post_complete')) ? eval($hook) : false;
            eval(print_standard_redirect('redirect_postthanks', true, $forceredirect));
        }

    } // end if

} 


As you can see the hook is located towards the bottom and I created a plug-in for that location and simply entered:

$vbulletin->url = 'my own url';

in the php code bit and it works fine.

However I am worried about the other bits of code in the original file above the hook location, so am wondering whether I need to add any of that in my plug-in? Or will what I have done only effect the line I want it to?

Sorry if its obvious!