What is the minimum setup to use VST on a live gig, hardware- and software-wise (please supply a complete list, including MIDI and audio stuff etc.)?
- Any system capable of running a plug-in host (PC, laptop, tablet, mobile).
- Audio and MIDI interface that is compatible with that system.
The minimal specs of the system will depend on the virtual instruments that you'll use. A simple subtractive synth will run in almost anything, but a robust sampler might need more RAM and disk space. You'll need to consult the minimum specs of your virtual instruments for this detail.
The minimal specs will also depend on how many instruments and processes you want to run at the same time. Unless you want to go nuts, something like Intel Core i5-4690K and 8GB 1600Mhz DDR3 RAM should be more than enough. You might need a lot less than this, but that setup would leave a reasonable room for expansion if you ever feel like going more crazy.
Test your vst setup with different systems, see if the systems can handle it (check latency, CPU usage, and if any artifacts are being induced), and write down the specs of those systems. This will give you a good idea of what you need and what to expect from different specs. You want your setup to run absolutely flawlessly: no artifacts, ultra low latency, and no CPU usage peaking near the limit.
Choosing the correct system for your needs (Mac, PC, Mobile, Laptop, OSX, Windows, Android, iOS, Linux, processor, RAM, etc) is crucial here.
Also, if you are very serious about this, you might want to consider having a backup system in case your main one fails. Consider having more than one if you can afford it.
Is there any software tailored specifically to live performances? I don't really care about recording.
There are many options, but I think the best one right now (and by far) is Ableton Live. It was tailored for live performance, everything is agile and intuitive. It is so much more than a plug-in host, so it leaves a lot of room for creativity. "Can we do this live?" With Live the answer is almost always "yes". Even if you imagine a device that doesn't exist, you can build it with MAX for Live.
One think that I love about Live for live performances is the Instrument Rack. You can layer many instruments in one single track, and have macros control different parameters from different instruments at once. This can be done in most plug-in hosts (in the form of multiple tracks and MIDI mapping), but nothing comes close to how fast, easy, and live-friendly Live implements it.
Check other options (BitWig and Mainstage come to my mind), but chances are that Ableton Live is exactly what you want.
Optimally, I would like to define presets for two keyboards in advance (e.g. upper keyboard = organ sound A, lower keyboard = piano sound B), and activate them with as little fuss as possible. How can this be done?
This is very easy to do, and most VST hosts can handle it (I can't think of one that wouldn't). Exactly how to do it will depend on your host, but it will almost always be more or less like this:
Make each keyboard send MIDI from a different channel, and make each virtual instrument receive MIDI only from the channel you want. Set the organ sound to receive MIDI only from channel 1, and set the piano sound to receive MIDI only from channel 2. Set upper keyboard to send MIDI through channel 1, and set lower keyboard to send MIDI through channel 2. Upper keyboard -> MIDI channel 1 -> organ sound, lower keyboard -> MIDI channel 2 -> piano sound.
To activate/deactivate you arm/unarm the MIDI track. You can do this from a control surface, if your keyboard has buttons that send MIDI you can have each keyboard have its own activate/deactivate button, which will be mapped to the arm/unarm function of the MIDI track.
How would you control the volume in such a setup? Dragging sliders with the mouse before your solo seems a bit impractical...
You can do it from a control surface. If your keyboards don't have knobs or sliders you can get a control surface with knobs and/or sliders, and assign them to whatever you want, including volume.
If you plan to have a lot of tracks, you'll make your life a lot easier by getting something with motorized faders. Something like Bheringer's BCF2000 or X-Touch.