I think the vibrams/landing on the balls of your heels idea is probably wrong. The main way one keeps their knees good while running is by having the hamstrings contracted a bit at the moment of landing. This keeps the knee joint in its tracking. Similar to how leg extensions are potentially bad for the knee joints because the lack of weight bearing can get the knee joint out of its tracking, but this can be remedied by simply contracting the hamstrings a bit while doing this exercise.

Furthermore, if one lands on their heels while keeping the muscles on the front side of the lower leg contracted, there is some cushioning also. Remember what the skeletal structure of the foot actually looks like. By landing on either the heel or the foot, but not right in the middle, there is some cushioning effect as long as the opposing muscle is working.

With landing on the ball of the foot though, it's a bit harder to have the hamstring contracted at landing, and there's just this bit of a braking motion which is hard on the knees as opposed to naturally rolling from the foot's heel to the foot's ball.

Yet, vibrams are so popular these days. So what do I know? Oh well.

...

Bill Toomey, the 1968 Olympic Decathlon Champion from the United States, made video tapes of 100 champion runners in Montreal in 1976 in order to evaluate their technique. It is common-sense to assume that the best athletes in the world (those who consistently place highly in major championships) share common technical traits. All of the athletes filmed by Toomey used the same basic technique - the same technique I use, and teach to my athletes. The athletes filmed by Toomey all landed on the forefoot. None of the 100 landed heel-first!

http://www.scribd.com/doc/18228/Gordon-Piries-Running-Fast-and-Injury-Free-ultimate-edition-240307
Not much actually.