When I've done dog shifting mechanisms before, I've set up the gear and the pocket so that when the side of dog touches the side of the pocket, the two faces are parallel.
Vex's CAD seems to be different. The side of their dog isn't parallel to the side of the gear's pocket, so the dog only transmits torque through one edge.
See the gap highlighted in yellow in this section view:
http://imgur.com/17hgx7y
Is this really how vex pro shifting dogs work?