Figured I would update my thread as some time has passed and I have used the PDX6/NFS mode as a backup device for over 4 months with VMWare, primarily using Veeam backup software. I have not had any re-occurences of the lost volume. The only issues I seem to have is re-connecting the NFS volumes after a power down of our environment. It could also be related to using the 802.3ad etherchannel connection (or both).
I only osbserved this re-mount issue when powering down our environment during testing of our power outage shutdown script, which when you are completely virtual has all types of nuances when starting up your environment again.
Since my original post I have:
1) Upgrade the firmware
2) Maxed out the drive capacity, for spreading the IO load accross more drives.
3) Converted to RAID6 as we have some development on the NAS too.