Sorry, I wasn't very clear: What I meant to say was that the mobile style in vBulletin 4.2 is not really fit for purpose; it doesn't work with any add-on (at least not intentionally, without explicit support being added) and today's web focuses on Responsive design rather than a mobile-specific style.
("Responsive design" means a style that adjusts itself based on the user's viewport, while maintaining the vast majority of functionality. This site uses responsive design.)
For that reason, the mobile style was not very popular. For evidence of this, you need look no further than the fact that no-one else has reported this issue (to my knowledge / recollection), in spite of the hundreds (if not thousands) of users that have used this product.
As far as I can remember, the only add-on that had explicit support for the mobile style was the Advanced User Tagging product for vBulletin 4. This received special attention because without said support, posts would not render correctly on the mobile style.
When you consider the fact that the mobile style is hardly ever used, and the fact that it would take significant time & resources to add support for it, there are no plans to add support for the mobile style to this or any other vBulletin product at this time.