Well, what do you get when you add black (RGB 0,0,0) to anything? Of course, you get whatever that anything originally was. So, the black outline was still there; it just wasn't having any effect anymore. Had the outline been white then it would have showed up.

So, yes, this isn't a deliberate intention of the add mode to eliminate outlines. It's an epiphenomenon (good word; look it up) created by using that mode in that particular circumstance. This is a problem with a software package as powerful as Vegas. It lets you tweak just about every parameter imaginable, and quite a few of these tweaks lead you into areas you don't expect. Is this a bad thing? NO! However, it does mean that one has to excercise caution and (heaven forbid) actually think once in a while. ;) If you want more foolproof editing take a look at something like Microsoft Movie Maker or Apple iMovie, both of which are limited enough that it's difficult to get into uncharted territory. If you want to expand your horizons and visualize your dreams, stick with the power of Vegas.