Thanks a lot I think the problem was with "nonce" as we indicated .

I am putting what worked for me for bittrex 1.1 api maybe it will be useful for someone else.

string apikey="xxx";
string apisecret="xxx";
string nonce =  Clock.currTime(UTC()).toUnixTime.to!string;
string url = "https://bittrex.com/api/v1.1/market/getopenorders?apikey=" ~  
                        apikey ~ "&nonce=" ~ nonce;
                        
auto hmac = HMAC!SHA512(apisecret.representation);
hmac.put(url.representation);
auto generatedHmac = hmac.finish();
string generatedHmacStr = std.digest.digest.toHexString(generatedHmac);
Json data;	
try
{
    requestHTTP(url.dup,
            (scope req) {
                    req.method = HTTPMethod.GET;
                    req.headers["apisign"] = generatedHmacStr;
            },
            (scope res) {
                    data = parseJsonString(res.bodyReader.readAllUTF8());
                    writeln(data);	
            }
    );
}
catch ( std.json.JSONException e )
{
        writeln("Exception was caught while authorization", e);
}

I am using vibe.d extensively last 2 weeks. I have some interesting non-blocking findings I will post them very soon.

Best Regards
Erdem