>>> (3) exceptionally in <vir>.
>> What would cause such an exception?
> Great frequency of use, most likely.
The vir word is itself a bit odd - some IE languages (e.g. Skt) show a long
i, others (eg Latin), a short. I don't see how that be connected with this
loss of final -us in any way, but it does mean there's stuff we don't know
about the history of this word.