CF has covered this in the 2002 forum review. The only time he recommends using a vest is during drills of up to an 8second duration. In the speed strength video he has the athlete doing running a’s and b’s while holding a 10kg weight plate. A weight vest would make it a lot more convenient doing these drills.
He also says that using a vest during sprints will cause the hips to lower causing footstrike to occur further ahead of the body which would increase ground contact times and increase the chance of a hamstring pull.