[@Jbcool] It's not really been discussed if there's an opposite end to the cat walk, connected to the far wall or not. If there was, though, there would be a good deal of space between Vespin and said wall, as he originally took up his current position to blanket-fire the door Oskar came in.

So, yes, he could potentially get behind Vespin. However, he wouldn't be able to do it quietly, by any means. Though, there's really not a great deal of need for it. If Decius has been listening in, he'd know that Vespin is quite literally offering them exactly what they're after; A path straight to the leader of the ship, a plan to take control of the command deck, a way to control the rest of the ship once they have the deck, and even a place to go and get a new ship when they abandon the one they're on.

That's exactly what he Just got done saying. That, and he's mentioned the traitor legions. Decius might be in a position to have issue with that, especially when Vespin just said 'You (the space marines) could all be Alpha Legion spies.'