Interesting!!! - I've confirmed the issue, but I'm surprised that this is a new problem.
http://stackoverflow.com/questions/2...-iphone-safari
https://discussions.apple.com/thread/3132659?tstart=0
First, scroll down a bit further on any of the pages on the site, and there is a "Top" link in the bottom footer (look in the way bottom right of the page). That is the one I was testing before on iOS 10.3.1, and it works fine in all browsers. The apparent problem is that the "Top" link in the middle of the page between the previous thread / next thread, is a dynamic reference to the top of the specific page, compared to just a generic link to the top of whatever page it is on. Desktop browsers aren't bothered by this. Mobile browsers apparently are. If you scroll over either link by the time it is interpreted in the desktop browser, they look identical.
I'm not sure if there is a fix, without messing around with core vBulletin code more than would be worth the effort/risk, but I am surprised that it has taken this long to be recognized. Keep in mind that much of the vBulletin code has been around for many years before the iPhone was dreamed up by Steve, so at times I'm somewhat surprised that it all still works as well as it does 15+ years later.