Can the vibrato effect activiated via a foot switch so that the user can control exactly when the vibrato kicks in and on what notes?
The vibrato style and amount are controllable by MIDI control change messages.
If you have a foot pedal (such as the TC Electronic G-Minor) or another configuration that can send standard MIDI Control Change commands, you can control these parameters.
You can send a value of 0 to the amount CC message for turning the vibrato off, and send a value between 1 and 127 to turn the vibrato on. This is essentially a depth control, so you send the depth you want each time.
Be aware that the vibrato looks for a stable note before it is applied, and some of the styles have a delayed onset. This can allow the note to sustain a bit before the vibrato comes in.