If you have access to Vcenter then you should have access to the console, true? You can then check your drives via the command line and using smartctl to see how they are doing and how the pool is doing via zpool status.
Here is the part which bothers me:
one particular drive gets SMART errors with impending failure and the freenas instance will lock up, vms and data are inaccessible and we only regain access once freenas and other vms are rebooted.
If this is a data drive then I don't understand how this could stop FreeNAS from running and your other VMs. You need to become very specific with the data you provide if you are indeed asking for help. If you cannot provide the information then it's very possible we cannot provide you good help.
Things you should post here are:
1) Is your memory locked for your VM for FreeNAS, 32GB you said above.
2) Which drive is giving you the failure (please don't say it's the SATA DOM, that would be too obvious).
3) If you are running ESXi and FreeNAS as a VM, why use a SATA DOM? Why not create a 10GB vmdk drive and put FreeNAS on that? That is what I do.
4) Post the SMART error message.
5) Post the complete SMART data for the suspect drive (smartctl /dev/adx -x) for example.
6) Post the output of "zpool status".
7) Post what version of FreeNAS you are using.
8) Post your hardware configuration (the actual hardware being used).
9) What version of ESXi are you using and what version of VM are you using?
10) Examine the log files for both ESXi and FreeNAS VM to see if there are any error messages of interest.
11) Run MemTest86+ on your system and a CPU test, maybe your system is not stable.
All of this data will help provide you the best possible help.
Question is, why is a single drive failure causing this, also the drives are SAS drives.
If the single drive failure is the boot DOM then it could be the issue. If you configured ESXi incorrectly this could be the issue. The suspect hard drive could be shorting out the power (pulling too much current) causing the power supply to drop the voltages being output and crashing the system. It's a guessing game at this point in time.