Here's what may be an odd one; I have talked to a techie at Zero G and he's says he's got no idea what's going on, but gets this question quite a bit.
I've got Vocaloid 4 which can run as a Vst or as Rewire. Neither shows up in Splat. An earlier version of Vocaloid did show up in X1 or X2 (can't remember which) as a Vst, but was never stable and I gave up the fight, working in the stand-alone Vocaloid and importing the generated wave files. This still works for Vocaloid 4, but I decided to check here to see if: 1) anyone has any experience with this issue, 2) was either a fix or work-around that worked and, 3) if there is a way to define ReWire devices if they don't show up (as I suspect that for this app, the ReWire may be more stable. This is just a hunch).
 
Any sort of suggestion will certainly be appreciated, but before anyone starts really trying to scratch their noggins about this, please understand that I'm not that bent out of shape about it. Importing works great and, as I'm sure many of you have experienced, the vocal generally gets added last to a composition for me anyway. So please, don't go crazy on my account. 
 
Thanks!

A critical part of rewire compatibility is that both rewire host and slave must be of the same bitrate. Meaning, if you're running Sonar 64 bit the other program also has to be 64 bit. No way around it. Also Sonar can only be the Rewire host not the slave. They are the only specifics i know of.

I believe SONAR reads HKEY_LOCAL_MACHINE\SOFTWARE\Propellerhead Software\ReWire to get the list of clients. The Vocaloid installer must register in that path to make SONAR's ReWire list.

mudgel
A critical part of rewire compatibility is that both rewire host and slave must be of the same bitrate. Meaning, if you're running Sonar 64 bit the other program also has to be 64 bit. No way around it. Also Sonar can only be the Rewire host not the slave. They are the only specifics i know of.

scook
I believe SONAR reads HKEY_LOCAL_MACHINE\SOFTWARE\Propellerhead Software\ReWire to get the list of clients. The Vocaloid installer must register in that path to make SONAR's ReWire list.

Well, it seems that Vocaloid 4 is installed in the Program File (x86) folder, but I'm still not sure that means it's 32 bit (I've seen a couple of 64 bit programs install themselves there, though I'm not sure why). I can email the techie to find out that for sure. Perhaps more tellingly, Vocaloid 4 is not installed under ReWire clients in registry. So, I have potentially two issues. Thanks, guys for alerting me to this. I'm not sure where I'm going with yet, but I have a couple of paths to check.