FYI: this behavior, the ability to set the height of the threaded mode view, was broken after we did the last vBulletin update.
About a week ago, we were able to partially fix it, so if you have a custom value in your profile settings for this, it will be applied. However, if you apply no size, it no longer automatically resizes to fit the number of posts there, instead, it uses the hard-coded 175 pixels.
I for example, set mine to 350, this way it's always 350, but it's much easier to deal with longer threads than the default of 175.