As a side-note:
The reason you must route any inputs ‘through the host’ is due to the ASIO protocol and standards. Plugins are not EVER supposed to access an audio device directly, as it can totally mess up timing synchronization in least, and cause buffer overruns and system crashes at the worst. I’ve only come across a few plugins that try to do this. They are hard to find, highly experimental, and tend to be very unstable (to the point that they can create such a noise they’ll blow speakers, and possibly damage your hearing).
If the host/DAW does not have a way to get the audio and route it into HALion, then you’re out of luck. This will be the case for any sampling instrument that adheres to ASIO protocol (I.E. Kontakt, Mach 5, etc.).
For a long time it was not possible to get audio ‘into’ a VSTi plugin at all. It could only be done with VST ‘effect’ plugins that were designed to go in an effect slot. Side-Chaining effect plugins was awkward as well, often requiring hacks like using a quad group bus and alternate routings.
VST3 protocol changes this, as VSTi instruments can have ‘inputs’ and support side-chaining. While it’s possible to make a VST2.4 ‘instrument’ plugin with inputs (The VST2.4 variant of HALion does have them), it’s not very common, as most hosts from that era would just ignore them if it knows it is an ‘instrument’ type rather than an ‘effect’ type plugin anyway.
Not all DAWs out there can do VST3 yet though. Not all of them support side-chaining like Cubase does, particularly for ‘instrument plugins’. Their workaround is that they ‘might’ allow you to host a VSTi in the effect slot of a group bus instead of providing methods to set up and route special ‘side chain’ buses. Some DAWs might pretty much do it that way out of the box anyway, and host instruments and VSTi plugins identically. Hosting something like HALion in such a group would provide a means to get audio into something like HALion.
For a DAW that doesn’t support Side Chaining, and also will not let you run a VSTi in an effect slot…you’re either out of luck, or must rely on some third party chainer that can fudge a VSTi into a regular VST effect slot (such as Bidule).
Ok, this explains why the standalone version does this easier than the VST do in Cubase9. That you for all your long explanations. I have managed to get it done with audiotrack and sidechain. Wish the manual was a little better to explain stuff like this.