I am having the same issue. I am not using VMware in my build but the symptoms are the same. When I try plug in SSDs, everything is fine until I try to add the 2 SSDs as SSD cache (added to Volume 1) and it crashes the volume. Only way to recover is to remove the SSD cache from the volume. Sometimes even that doesnt work.
Model Name: DS3612xs (Nanoboot)
DSM Version: DSM 5.0-4493 Update 1
I wish this feature worked! I need more IOPS very badly in order to run my VMware environment.