#!/usr/bin/env dub
/+ dub.sdl:
	name "test"
	dependency "vibe-d" version="~>0.8.5"
+/

import std.stdio;
import vibe.d;

void main() {
   HTTPClientSettings settings = new HTTPClientSettings;
   settings.proxyURL = URL.parse("http://gb.smartproxy.com:30000");
   auto response = requestHTTP("https://dlang.org", (scope req) {
       req.method = HTTPMethod.GET;
   }, settings).bodyReader.readAllUTF8;

   writeln(response);
}

gives: vibe.http.common.HTTPStatusException@/home/tobias/.dub/packages/vibe-d-0.8.5/vibe-d/http/vibe/http/client.d(416): Proxy Authentication Required.

Sorry, can't dig any deeper.