Strange. This example crashes (Windows 7 64bit, dmd 2.063.2, driver: libevent): 

import vibe.d;

shared static this() {
    runTask({
        sleep(1.seconds);
        listenTCP(8000, (conn){
            accept(conn);
        }, "127.0.0.1");
        auto conn = connectTCP("127.0.0.1", 8000);
        logInfo("client: connected");
        conn.write("client: hello");
        logInfo("client: sent");
        ubyte[13] buf;
        conn.read(buf);
        logInfo("client: received '%s'", cast(string)buf);
    });
}
void accept(TCPConnection conn) {
    // Trying send and receive in new task
    runTask({
        conn.write("server: hello");
        logInfo("server: sent");
        logInfo("server: accepted");
        ubyte[13] buf;
        conn.read(buf);
        logInfo("server: received '%s'", cast(string)buf);
    });
}

package.json: 

{
    "name": "test",
    "dependencies": {
        "vibe-d":       "~master",
    }
}

If to comment runTask in accept functions, assertion disappears.