On re-reading source material, I don't remember seeing anything stating what the tobermoon's damage was like. I don't think it was the Ur-Quan. If it had been, unzervalt would have been enslaved much sooner and the ship would be space debris. Note that the ship is still well enough to travel in hyperspace and that no bodies are found on board.
It sounds eerily similar to the disappearance of the Androsynth, and Vela isn't too far from Orz space, but it's well before most of the IDF experiments, so that seems unlikely. I also have a completely random theory that it was the Arilou protecting the greater cause of Humanity by keeping Vela's secret.
Mmrnmhrm 15:58, 21 Oct 2004 (CEST)
I seem to remember the SC2 for PC manual implying that the Tobermoon was attacked by an Ur-quan Dreadnought and the crew enslaved. They might have changed it, I guess.
Actually, both SC2 manual (95% sure - will check) and capt. Hayes mention a red unmanned probe killin capt. Burton (it is mentioned in the remark about multiplying probes, I think). This clearly points to the Slylandro.