I would rule that you target both the speeder and any exposed passenger/driver within 6 square cone as this is in the description of the force power.
So, what happens if the attack beat the threshold of the speeder? I think you could just use the same rules as for a creature, the speeder comes to a stop and the driver must spend a move action to get the speeder ready to move again.
What about hitting the threshold of the driver or passenger? Well, they go prone. I would rule that you can't pilot a vehicle while prone, but that would be my interpretation. A driver would have to spend one Move action to get upright and one Move action to control the speeder. If the driver for some reason can't spend a second Move action to maintain control, he loses control over the vehicle and it moves its speed in a straight line and then comes to a stop. If there is any obstacle in front of the vehicle this may result in a crash.
Force Thrust is much better for throwing people of vehicles as it has the potential to move the target several squares...