Hmm, I see it's the charged beam the battlecrab hits,now that you mention it.
However, I looked carefully, many times, on the next shot, and you cant really see the beam penetrating the Vorlon ship. And when the cruiser fly's by, you cant see any damage on the other side of the Vorlon's claw that was hit, though it wouldn't be easy to spot anyhow.
Oh well, one learns something new every day, if one pay attention
However, I still think the Vorlon ships are clearly superior to that of other races, and they did manage to beat back the third space aliens, even though some of their own was telepathically controlled by them.