Well, if we are only to look at Accelerator's esper ability, controlling vectors, I think it's actually more defined than Shiki's "Mystic Eyes of Death Perception".
Reason is, in Accelerator's case the question is like: "Does it have a vector?" if so, he can control said vector. Well, you can still use his powers against him, like Kihara did, but overall his powers are less complicated.
Accelerator's power is more of a yes/no question. If it has vectors, Accelerator can control them and that's it.
Shiki's power involves a lot of other variables. We have no idea when his eyes will have an overload and what he can "kill", since "killing" poison left him hospitalized for a lot of time in Kohaku's route.
There are still other questions to Accelerator, of course, and there's more to him than vector control... But, I feel like his ability is more concrete than Shiki's, at the very least. If you decide to take Accelerator more deeply and look at him like an Artificial Angel, I guess things get a little bit more complicated, but "Vector Control" is doubtlessly less vague than "Mystic Eyes of Death Perception".
Btw, my favorite Level 4 is Kuroko.