How to log in to a vbulletin forum with C#?

Hi,I'm trying to log into a vbulletin forum. I got this far:

PHP:

private string login(string url, string username, string password){string values = "vb_login_username={0}&vb_login_password={1}"values += "&securitytoken=guest&cookieuser=checked&do=login";values = string.Format(values, username, password);HttpWebRequest req = (HttpWebRequest)WebRequest.Create(url);CookieContainer a = new CookieContainer();req.CookieContainer = a;System.Net.ServicePointManager.Expect100Continue = false; // prevents 417 errorusing (StreamWriter writer = new StreamWriter(req.GetRequestStream(), System.Text.Encoding.ASCII)) { writer.Write(values); }this.response = (HttpWebResponse)req.GetResponse(); StringBuilder output = new StringBuilder();foreach (var cookie in response.Cookies){output.Append(cookie.ToString());output.Append(";");}return output.ToString();}