Module.vb

Sub Main()
    ' ログイン（Cookie の取得）
    Dim session = GetUserSession("メールアドレス", "パスワード")

    ' マイページ取得（取得した Cookie を付けてアクセス）
    Dim url = "http://www.nicovideo.jp/my/top"
    Dim req = WebRequest.CreateHttp(New Uri(url))
    req.CookieContainer = New CookieContainer
    req.CookieContainer.Add(session)

    Dim html As String
    Using res = req.GetResponse
        Using rs = res.GetResponseStream
            Using sr = New StreamReader(rs)
                html = sr.ReadToEnd
            End Using
        End Using
    End Using
End Sub

Private Function GetUserSession(ByVal email As String, ByVal password As String) As Cookie
    Const path As String = "https://secure.nicovideo.jp/secure/login?site=niconico"
    Dim req = WebRequest.CreateHttp(New Uri(path))
    Dim param = String.Format("mail={0}&password={1}", Uri.EscapeDataString(email), Uri.EscapeDataString(password))
    Dim buf = System.Text.Encoding.UTF8.GetBytes(param)

    req.Method = "POST"
    req.ContentType = "application/x-www-form-urlencoded"
    req.ContentLength = buf.Length
    req.Referer = "https://secure.nicovideo.jp/secure/login_form"
    req.CookieContainer = New CookieContainer
    Using rs = req.GetRequestStream()
        rs.Write(buf, 0, buf.Length)
    End Using

    Dim html As String
    Using res = req.GetResponse
        Using rs = res.GetResponseStream
            Using sr = New StreamReader(rs)
                html = sr.ReadToEnd
            End Using
        End Using
        Dim cookies = req.CookieContainer.GetCookies(New Uri(path))
        Return cookies("user_session") ' MEMO: 無い場合は Nothing
    End Using
End Function