Funny you should ask, I've run across the same thing and have asked about it several times of different vegas forums, to no avail. My problem occured when I added undersampling and velosity envelopes to material to make it look like a Charlie Chaplin movie. Did you happen to do anything of this sort to your material? ALso, I could see the blurr immediately when I previewed at GOOD or BEST. Do you notice that as well.
Eventually I just rendered at preview and called it a day.