Vereen's a tough "little" guy (205 is little on that level, I guess) with a quick enough start to neutralize the initial charge of all but the quickest linemen. Not for long, but enough. And misdirection is the name of the game when teams are pretty evenly matched.
Using a running back as an ineligible offensive lineman is a horrible idea, his speed is virtually worthless for blocking, and a charging linebacker will knock him backwards. Remember, a running back has a running start when he hits a linebacker. Give the running start to the linebacker and the advantage shifts, dramatically. Vereen vs. a charging Clay Matthews: no contest.