http://cormachogan.com/2015/02/24/vsan-considerations-when-booting-from-usbsd/
This clears it up but does not mention viable solutions.
In my opinion now I need to go back to using use two mirrored physical disks for VSAN and vmkernel logs; VSAN trace files; capturing core dumps (PSOD).
vSphere 6.0 Documentation Center
https://kb.vmware.com/kb/1033696
Thoughts
Jeff