Ah, just found out something. In virtual Audio Cable, the checkbox "Volume control" can be enabled. Then, I select "Virtual Cable 1" in the Volumouse Options. Then I can control the volume with the mouse wheel or a dedicated controller acting as a mouse wheel.
Still the need for a VST Volume Control remains once I wanna go the bit-perfect and/or kernel streaming route, bypassing the kmixer, using foobar2000.