VST are virtual "instruments", stored in a file.
Many VST are available on Internet. Latency of the VST is a critical parameter in the Expresseur. The global latency (audio + VSTi + VSTe) must be less than 30 ms to have a reactive playing.

There are two main families in VST :

Warning : some VST need lot of resources. The performance of the PC can be a critical parameter to have low latency.

Majority of VST have Graphic User Interface. To view the VST interface, select menu View / VST.

VST window has a small menu to save and load VST configuration, using FXP format. These files can be used dynamicly in the Tunings.

For VST which does not respond to Midi Control messages, you can use processes to inject directly programs or parameters.

Note : VST are not yet available on Mac.
To use VSTi (or any other MIDI software) on Mac :