Just an update:
After spending some time with VMware technical support, we determined the quickest way to resolution that was "clean" and didn't involve and outage window of the entire environment was just rebuilding the VSA. A root cause was never determined, however there a few issues when fsck was run, so possible corruption of some sort.
I spent the time to move the VM's to local diskspace, tear down the VSA and rebuild and it seems OK. However a storage increase was not attempted again.