Good evening, scratch should not go on the vSAN datastore. Ideally your SD or USB card would be big enough. If not you can use a local datastore on each host. That's what we had to do, take a disk out of a disk group and make it a local datastore. Not ideal as there is no redundancy.
Also, there are considerations for the vSAN trace files. http://cormachogan.com/2015/08/14/handling-vsan-trace-files-when-esxi-boots-from-a-flash-device/
Thank you, Zach.