Those connections you saw in my first screenshot are in a software called “Cadence” which has more or less the same purpose as “aconnect” or QJackCtl.
When trying to monitor the MIDI messages with something else (kmidimon, aseqdump) than SuperCollider I don’t receive anything.
Will try to contact the VMPK developer(s).
So if anyone else using VMPK could reproduce the problem that would already be interesting for me.
edit: Problem was with something vmpk-related.