Recording software instruments using plugin's keyboard

[Geetar]

Hey all, it's been forever since I've tried to record any midi without a controller, but I find myself this morning trying to get an arpeggio from Vacuum recorded into a track just by using the plugin's on-screen keyboard but it's not recording any midi into into the track. Do I need to enable something that i'm missing here?

I've got the software instrument track record enabled, vacuum is loaded and I can hear the audio coming through when I record, but no midi info is showing up in the region.

Probably something really simple - appreciate the help!