The duration of Vermin shape if 1min/level, but it lacks that little (D) notation, so it can't be dismissed.
Is this intentional?
I certainly hope not.
It would be rather odd if it was the only beneficial polymorph spell that can't be dismissed.
Is there an errata somewhere I have missed?
I don't see any errata or FAQ covering this.
It could be ended early by being subject to another Polymorph effect, but that's not optimal for in-game use.