vibe.d is blocking threads

[RuZzz via Digitalmars-d-learn]

Code:
    import std.concurrency;
    import core.thread;
    //import vibe.http.client;      // If uncommented this line, 
the thread "worker" does not start
    void worker() {
        foreach (i; 0 .. 5) {
            Thread.sleep(500.msecs);
            writeln(i, " (worker)");
        }
    }
    void main() {
        spawn(&worker);

        foreach (i; 0 .. 5) {
            Thread.sleep(300.msecs);
            writeln(i, " (main)");
        }

        writeln("main is done.");
    }

How to launch threads with vibe.d? It doesn't work at both 
compilers.

- Gentoo Linux 64
- DMD64 D Compiler v2.069.0 (dub dependency)
- DMD64 D Compiler v2.071.0
- dub-0.9.24 USE="dmd-2_069"

dub.json:
{
  "name" : "axt-threads-json",
  "description" : "A minimal D bundle.",
  "targetType": "executable",
  "targetPath": "bin",
  "dependencies" : {
      "vibe-d": "~>0.7.28",
  },
  "configurations": [
            {
                "name": "Debuger"
            },
            {
                "name": "Release"
            }
    ]
}