I'm trying to use a MIDI USB keyboard (Akai LPK25) for MIDI instruments and it does not seem to play nice with either Sonar or the VS-100.
 
Sonar sees the LPK25 under MIDI Devices and I can click the box for the LPK25 under Inputs, but the setting doesn't seem to commit.  When I go back into MIDI Devices, the box is unchecked again.

Any thoughts?

I'm having the exact same problem.  Did you ever figure it out?

I did have this unit before I upgraded to the m audio oxygen 49..its a little thing right?
worked awesome for me, never had an issue.
I know that probably doesn't help here but I will say when I went to trade it in the idiots at GC couldn't figure out how to use it as a midi device.

anyway, I installed the drivers /software that came with it, I know its a plug and play but there is a disk tht sold with it.
 I installed that and it worked great.

make sure input echo is checked.
make sure it is selected under devices (you did)
make sure its selected as your input on your midi track (the vsti your triggering)
in other words, which ever vsti you are using it for(example dimension pro) make sure dimension pro's input is selected to lpk25.

if your losing it its most likely because of the midi input on your vs 100.
but this is a usb device. so install the hardware that came with it first.
then check it under midi devices, remember its not a control surface so no need to check it in there.
just under devices in and out.

thank me later when its working.

and remember this...its very important.

*always.. and I mean always plug you vs 100 and lpk25 in to the same ports you installed the software driver in.
in other words, you pluged the vs 100 in to the front port # 1 all the way to the left (on a desktop)
and you installed the drivers with it plugged in to this port.

then always plug in to this same port...the one you plugged it in to when you installed the drivers.
your device may...but it wont work properly.

if you did install software for the lpk25 plug into the same port you installed the software to.

this wasn't as important back in windows xp days, but win 7, 8 and even vista have assignments for drivers/ ports.

*always.. and I mean always plug you vs 100 and lpk25 in to the same ports you installed the software driver in.

 
I also use the VS-100 and an Oxygen 49.  No problems so long as I don't change USB ports.
 
Glen