Simple VAC Ban checker. You can give custom url (ardaozkal) or steamid64 (76561198034299068) to it and it'll return true if the user is vac banned (or if the account is unable to be found).

VacBanCheck.cs

        static bool IsVacBanned(string steamID)
        {
            var wc = new WebClient();
            string ProfileLink = "http://steamcommunity.com/";
            ProfileLink += (steamID.StartsWith("765")) ? "profiles/" : "id/";
            ProfileLink += steamID + "?xml=1";

            var ProfileData = wc.DownloadString(ProfileLink);
            return !ProfileData.Contains("<vacBanned>0</vacBanned>");
        }

You can also do "using (var wc = new WebClient())".