Handling Patch Changes in VSTs --

Yes, soft synths can do that, too, and it works just like it does in the hardware world.

However, we often use the term "soft synth" to apply to devices that are not really synthesizers in the conventional sense but are actually sample players. Unlike a ROMpler hardware synth, the samples are typically not held in memory for quick selection (too big) and instead have to be loaded from disk as needed.

A 20-second load time might be acceptable in the studio but it feels like eons onstage. Plus you have to initiate it from the laptop, using a mouse or (ugh) touchpad -- in the dark, reading a too-small UI. Hopefully, you're not the one who's expected to provide audience smalltalk to avoid dead air between songs.

One way to get around that is to use a multi-timbral synth/sampler, load up all your patches in advance and then select the desired patch, not via a conventional MIDI patch change sequence but rather by MIDI channel selection. That way, you can set up the laptop once and then won't need to touch it again during the performance.

Anyhow, that's how I would do it - if I was brave enough to actually incorporate a laptop into my live rig.