Error with SUNAT HttpWebRequest {WebService: "Error on the remote server: (500) Internal server error."}

I'm trying to consume a WebService but I have the following error:

  

{"Error on the remote server: (500) Internal server error."};

The address URL of WebService is the following link .

Through a Reference I could solve it, what I'm looking for is to do it with code .

 public string EnviarXML()
    {
        string vResult = "";
        string cFileNameZip = Path.GetFileName(cRutaFileZip);
        string cfilebyt = Convert.ToBase64String(File.ReadAllBytes(cRutaFileZip));
        byte[] vRequestDataXml = CreateHttpRequestXmlEnvio(cFileNameZip, cfilebyt);
        vResult = CallWebMethod(vRequestDataXml);
        return vResult;
    }
    public string CallWebMethod(byte[] valRequestData)
    {
        try
        {
            //byte[] vRequestData = valRequestData;
            HttpWebRequest vhttpRequest = (HttpWebRequest)HttpWebRequest.Create(cWebServiceURL);
            vhttpRequest.AllowWriteStreamBuffering = false;
            vhttpRequest.Headers.Add("SOAPAction", "sendBill");
            vhttpRequest.Method = "POST";
            vhttpRequest.ContentType = "text/xml; charset=UTF-8";
            vhttpRequest.Accept = "text/xml"; 
            vhttpRequest.ContentLength = valRequestData.Length;
            vhttpRequest.ProtocolVersion = HttpVersion.Version11;
            vhttpRequest.Credentials = CredentialCache.DefaultCredentials;
            vhttpRequest.Timeout = 30000;
            vhttpRequest.UserAgent = "Mozilla/4.0 (compatible; MSIE 6.0; MS Web Services Client Protocol 4.0.30319.17929)";  
            vhttpRequest.GetRequestStream().Write(valRequestData, 0, valRequestData.Length);
            string vResponse = string.Empty;
            HttpWebResponse vhttpResponse = null;
            vhttpResponse = (HttpWebResponse)vhttpRequest.GetResponse();
            System.IO.Stream vbaseStream = vhttpResponse.GetResponseStream();
            System.IO.StreamReader vResponseStreamReader = new System.IO.StreamReader(vbaseStream);
            vResponse = vResponseStreamReader.ReadToEnd();
            vResponseStreamReader.Close();
            return vResponse;
        }
        catch (Exception ex)
        {
            throw new Exception(ex.Message);
        }

    }
    private byte[] CreateHttpRequestXmlEnvio(string cFileName, string cFileBase64)
    {
        UTF8Encoding vEncoding = new UTF8Encoding();
        string vxml = "<soapenv:Envelope xmlns:ser=\"http://service.sunat.gob.pe/\" xmlns:soapenv=\"http://schemas.xmlsoap.org/soap/envelope/\" xmlns:wsse=\"http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd\">"+
            "<soapenv:Header>"+
                "<wsse:Security>"+
                    "<wsse:UsernameToken>"+
                        "<wsse:Username>" + cUserSol + "</wsse:Username>" +
                        "<wsse:Password>" + cPwdSol + "</wsse:Password>"+
                    "</wsse:UsernameToken>"+
                "</wsse:Security>"+
            "</soapenv:Header>"+
            "<soapenv:Body>"+
                "<ser:sendBill>"+
                    "<fileName>" + cFileName + "</fileName>"+
                    "<contentFile>" + cFileBase64 + "</contentFile>"+
                "</ser:sendBill>"+
            "</soapenv:Body>"+              
        "</soapenv:Envelope>";

        return vEncoding.GetBytes(vxml);
    }