So, the idea has been going around that cross-synth is legal for personal use. I actually thought it was too, however looking at the evidence provided. The section they cited was actually referring to who could use Vocaloid. Here is what they claim to be permission to use cross-synth between vocaloids.
At a glance it seems like it's okay, but here is what I failed to do: look at the context. It is not talking about modifying the software it is talking about allowing unlicensed users to use your own copy of the software:
You may not use the Software and Voice Library in a mode of operation where the Software and Voice Library are unattended by an operator for any purpose whatsoever, other than for personal use. …Read more >