I looked up the weak spots fot Reach's vehicles.
The Scorpion's weakest spot is the vent on its back, the damage is tripled there. One Laser kills it, one rocket, a full sniper clip, and 3 plasma launcher shots.
The Wraith is obviously the power core thing, where damage is quadrupled. One Laser kills it, two rockets (for whatever reason), one sniper shot, and two plasma launcher shots.