If I'm not mistaken, the main people Oskar wants dead are actual, hard-core criminals. Vespin wouldn't be killing them to befriend Oskar; he'd be doing it to protect his own men.

Remember, at present, Vespin is still loyal to the guard. It's a group of ratlings that were sent to deal with the prisoner escape. That's going to mean him loosing allies, customers, and possibly even friends if things go sideways. He intends to avoid that.

The plan is going to be for him to use a superior knowledge of the ship's interior to chart a shorter route than the one Oskar is taking to get to the armory first. It's feasible that he could, given a wider array of routes to take, because he doesn't have to stick to the ones big enough to accommodate a prison's worth of men and is half the size of most of them to begin with.

Once there, he's going to use what little time he has to find an autogun turret with a bullet shield, set it to single-fire, and start splattering criminal across the wall as soon as they cross the threshold. Ideally, after killing the criminals he knows from his connections are bad news, he'll announce himself to try and ease tensions with the group by talking them down through his connection to some of them. One of those people he'd have a (albeit shaky) connection to is Oskar.