Does SoftNAS support VMware vMotion?
Yes, vMotion is proactive tool for scheduled maintenance and it will take a while to migrate depends size of the VM.
Vmware HA or FT should use for hardware or infrastructure failure. HA is restarting to another ESX server and FT creates a mirror vm in other ESX host as cluster which gives you zero downtime but does have limit on number of vCPU.
For FT, Vmware version 5.x support only 1 vCPU. In version 6, Standard and Enterprise supports up to 2 vCPU and Enterprise Plus support up to 4 vCPU.