I wanted to create a custom console logger to extend the functionality provided. I found that I couldn't deregister the standard logger because I couldn't get a reference to the logger instances.
I ended up having to edit the core files...

source/vibe/core/log.d

private shared(Logger)[] getLoggers() nothrow @trusted { return ss_loggers; }

became:

shared(Logger)[] getLoggers() nothrow @trusted { return ss_loggers; }

source/app.d

    auto loggers = getLoggers();
    foreach(i; loggers) {
        deregisterLogger(i);
    }

Seems to me that the getLoggers function shouldn't be private, or there should be a way to deregister all loggers.