It's not a stupid question IMHO because I was a vbulletin user for years without realizing that the editor has two modes, like you described. You can switch between them by clicking the button that looks kind of like A/A (actually, it looks exactly like this:

). It's in the upper left corner in the latest version of vb (4.1.4), and in the upper right in the older versions. I believe there's a way to select which is the default mode for users, but I can't seem to find it.