If you put copies of the video clip on tracks 1 and 2, so that they are synchronised, you can apply a blur to the top one, and then crop it so that the blur affects only the mouth area. With the bezier mask feature in Vegas 5, you can keyframe the cropped area and move it around to follow any movement of the subject' face. You can also apply a nice feathered edge if you want.
Tell the guy to speak very very very fast then slow down the picture...
Naaaah... I'd go with all the above and if you still got Vegas 4, you can use the cookie cutter to key the mouth instead of bezier mask.
It's not as complicated as it might seem when you get down to it... : )
we made a hilarious music video, and the dude said THREE where he should have said FOUR. haha. so to be even funnier, we just are going to blur it out.