Can I connect VB3-II to an external rotary speaker?

Of course! All you have to do is to turn the internal simulation off and adjust the other parameters (overdrive, keyclick, etc.) so that you always have the best tone that gets into your real rotary cabinet. Also, some vintage rotary speakers need a very hot signal, in this case you'd maybe need an external preamplifier between your sound card's output and your speaker's input.