Surely releasing the data cannot be more "vexatious" than the arduousness of having to go through trials and appeals. It
Is the term "vexatious" still being used? If so, it would be necessary to question whether it is used in an everyday sense to mean "annoying" or "tiresome" or "burdensome", or whether it is used in a legal or quasi legal sense to mean "persisting with an unarguable case to an unreasonable extent", or something of that sort. I have not looked up the definition, but it must be along those lines. Given the context one would expect the legalistic meaning to be intended.
It is suspected that the term is used in the first sense, but with the hope of giving it the aura of the second. It can be so much more effective a term if those against whom it is used can be compared to "vexatious litigants"
It would seem to be an untenable argument that an argument upheld by the Information Commissioner could be regarded as untenable or unreasonable, or, indeed, vexatious.