php - Get vsprintf() to set 3rd argument / lastname

In my serialize() I have 3 arguments. Member id, firtname, and lastname

a:3:{s:9:"member_id";s:1:"1";s:9:"firstname";s:7:"John";s:8:"lastname";s:12:"Doe";}
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As you can see it only out puts the first name

Current Output

<a href="http://www.example.com/admin/member/profile/?member_id=1">John</a>
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What it should be

<a href="http://www.example.com/admin/member/profile/?member_id=1">John Doe</a>
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Question: How can I make sure can get last name as well in vsprintf()

public function test() {

    $this->data['logs'] = array();

    $logs = $this->log_model->getlogs();

    foreach ($logs as $log) {
        $comment = vsprintf('<a href="member_id=%d">%s</a> logged in.', unserialize($log['log_text']));

        $find = array(
            'member_id'
        );

        $replace = array(
            site_url('admin/member/profile/?member_id')
        );

        $this->data['logs'][] = array(
            'comment' => str_replace($find, $replace, $comment),
            'date_added' => date("F jS, Y H:i:s A", $log['log_date'])
        );
    }
}
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