vBulletin has different options for determining how it marks forums as read. As you might recall from about a year ago, we have it set to automatically mark this information in the database. This can be resource heavy, but it's the most accurate. Since not all forums use this method (because of resource constraints) the application was built such that it uses it's own method of remembering where you've read in a thread. If you go to a thread that you haven't been to before in the app, it doesn't know where you left off so it puts you at the first post. If you open a thread that you have read before on the app, it should deposit you where it thinks you left off. This can be an inconsistent user experience, and the development team understands this. They are working on an appropriate solution.