Not to turn this into a discussion that should be moved over to Trek Tech, but since it's been stated in canon that Romulan ships are powered by an artificial quantum singularity (I am operating under the assumption here that somehow they have "harnessed" an artificial black hole), could it simply be that the Verithrax just got close enough to the cube to ram it and that the cube was drawn into the mini-black hole?
No way. A quantum singularity is microscopic, extremely low in mass. It can only "draw in" things that get extremely close to it. You could probably send a quantum-singularity "bullet" clear through a Borg cube, but the damage would be little worse than a micrometeoroid would inflict, just a bunch of small puncture holes. And that's assuming a comparatively large quantum singularity, large enough to be useful as a power source (assuming the power is generated by dumping stuff in and harnessing the resultant x-ray emissions -- which calls for a singularity large enough to draw in more than the occasional subatomic particle that gets too close).