I don't use Tunelab, but I remember there was a discussion about some of its extensions (with not so legal Vocaloid5 engine files):
https://vocaverse.network/threads/warning-some-tunelab-extensions-may-be-illegal.9264/
About using or not using, it more depends on you. It's unlikely that Yamaha will track your usage and ban your Vocaloid account, but it's possible that some original songs or covers "might" be tracked and blocked.
Again, because I don't use Tunelab, I really don't know if its software goes against Terms of Service of Yamaha, but if you want to don't have any worries, just keep using original Vocaloid6 (and future Vocaloid7) editor and supported DAWs like Cubase and FL Studio.