The Vaadwaur were a pretty interesting villain race and were setup for another appearance but then completely forgotten. It was kind of weird honestly. One of VOY's more glaring ball drops in my mind.
I don't know why... but I do wonder if their creation was part of it. That was going to be a two parter that introduced them, then they decided it didn't work and made it one episode.
I wonder if that painful inception put the writers off... or in the process of going from two to one episode, they realised they didn't know quite what to do with them.