VPAT is a statement by the authors of software, showing how accessibility needs are taken account of in the software. Buyers and users of the software may wish to (or have a duty to) take that into account in their decision whether they will use the software.
WMF might be asked for Mediawiki's VPAT statement, as the developer of the Mediawiki software, by anyone who wants to use Mediawiki and wants to (or needs to) take into account its accessibility standing, either for policy reasons or because they are under some kind of obligation (eg legal requirement) to do so.
The existence of a VPAT might be of general interest (eg on mediawiki-l), but a request or discussion by a specific potential Mediawiki user making an inquiry isn't really a list matter. It's more an administrative inquiry.
As an example, here's Mozilla's VPAT for the Firefox browser:
http://www.mozilla.com/en-US/firefox/vpat-3.html and http://www.mozilla.org/access/section508
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