In my experience with dynamic driver based IEMs the vented ones have consistently better sound stage. Could be coincidence, but I doubt it. The vented Final Audio BAs have very good soundstage, better than other single BAs I've heard (e.g. X10, PL50). Again, could be coincidence, but they also have surprisingly good texture for BAs, which should help with soundstage.
Maybe the causal relation is: venting => better texture => better soundstage. Speculation of course, but there's enough evidence among the phones I've heard to support the idea.