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. You may not use or have your clients use the Software and Voice Library as part of recording studio or rental company services, as each person who uses the sounds must be a licensed user.
The part that is really referring to modifying or changing the software really has this to say:
You may not engage in reverse engineering, disassembling, decompiling or otherwise deriving a source code form of the Software and Voice Library by any method whatsoever, and may not reproduce, modify, change, rent, lease, or distribute the Software and Voice Library in whole or in part, or create derivative works thereof.
Cross-synthing between different singers, is completely illegal, and really, may just work with Pocaloid for all we know. Also, even if cross-synthing between singers was only for personal use, putting it on the internet for the public to see is not personal.
For the same evidence in the V2 license, check here.