I just discovered a few posts calling out a bug in the vSphere 5.1 vCenter performance statistic rollup procedure for performance statistics. Any performance statistics greater than 30 days are lost. VMware does not currently have a resolution, but I am sure they are working on this.
I consider this a major bug as this information should be used by VMware administrators for capacity projection and baseline analysis if they already do not own a separate product that does this for them.
Keep tuned to VMware KB 2042164 for more details.