Hi Junichi, thanks for your interest.
  I might be missing something but is there something
that shelljam does
 that vkeybd cannot do? 
Yes.
* Shelljam is designed from the ground up to be a full-fledged musical
  instrument that can be used for professional live performance.
    -Safe, locks keyboard
    -Uses a compiled language
    -Performance is measured as latency and jitter
* Shelljam is portable, flexible and extensible
    -The I/O library supports, additionally to the keyboard under X,
     console keyboards, mice, joysticks, touchscreens, Sharp Zaurus
     and iPaq input, and all PC operating systems.
    -The MIDI library supports all PC operating systems
    -Instruments can be created as run-time libraries
* Shelljam has bold plans for the future:
    -Create a tool that can be used by a whole movement of kids,
     designed from the ground up to use the computers they have in
     their homes as full-fledged, highly tuned musical instruments.
    -Shelljam will support not only keyboards but mice, joysticks, and
     linux handheld devices to make music in genuinely new ways
    -The author intends to make a living by being a singer/computer
     peripheral player
Shelljam has a slightly higher learning curve and currently the UI looks
like you just crashed your computer so if you just want to test your
synth by all means use vkeybd.
Carlo