Okay, now... there's random chance, there's luck, and then there's just damn impossible.

Vespin had a map, was running with practically nothing on him, and went through vents and shafts that only a ratling would be small enough to get through to get ahead of Oscar and the prisoners, on an even higher floor. He didn't even take the same path Oskar's men did from the prison, because it didn't go where he was planning to get to.

The foot of Cuddles, the Holy Emperor's own pet rabbit, wouldn't have the luck you'd need to get a tech-marine in damaged armor behind Vespin within any reasonable amount of time for his arrival to have any affect on the conversation. The commissar with the horde of ratlings would have showed up before the tech-marine would have gotten behind Vespin.

Also, Vespin is a ratling, a trained sniper, and deals with thieves and other criminals on a regular basis. A space marine in fatigues would have a hard time sneaking up on him on a catwalk, coming out of a metal hallway. How did one wearing tech-marine armor pull it off, when he has no reason to be quiet?