I launch qjackctl, then qsynth, then qsampler, then vmpk. vmpk sees fluidsynth in the MIDI connections dialog but it doesn't see linuxsampler.
The workaround is easy: make the MIDI connection in the ALSA tab of qjackctl and it works fine.
VMPK (using RtMIDI functions) enumerates and filters the existing ALSA sequencer ports in the system having the port type flag SND_SEQ_PORT_TYPE_MIDI_GENERIC, that means "This port understands MIDI
messages." according to the alsa library documentation:
This is the same policy used for instance by aplaymidi, from alsa-utils. If you execute "aplaymidi --list", it doesn't show linuxsampler either. LinuxSampler should add SND_SEQ_PORT_TYPE_MIDI_GENERIC to their sequencer ports.
After so many years, this bug can be closed now that v0.6.0 has been released, because Drumstick RT does not filter the available connections like RtMIDI did. So finally, you can see linuxsampler in the connections list of VMPK.
Anyway, linuxsampler should still fix their ALSA sequencer port flags, because other RtMIDI users may suffer this issue.