Masking Inconsistency?

First, please accept my apologies for being new to Sony Vegas Pro 11 (or any version for that matter) and also to Creative Cow but thanks for any help or consideration of this matter of mine.

I am attempting the old Conan O'Brien gag of super-imposing a live action mouth onto a still image. Sony has a great webinar on the topic and I could follow it perfectly with some test footage I imported into Vegas Pro. So here's my issue...

I used the top track for my live action mouth and used the Cookie Cutter Video FX to mask out all but an oval shape over the mouth. On track 2 was the still image of what the mouth would appear over. That worked fine. However, when I added tracks 3 and 4 to do the same thing with two other video tracks, I couldn't select the oval shape. The default was Circle and although I selected Oval, only the Circle shaped mask would show. No other shaped mask would work either.

Is this a glitch or am I doing something wrong? I assume the latter...