There is a means to set a "Flash read cache reservation" on a hybrid VSAN so a VMDK can in essence "live" in the caching tier on a hybrid configuration.
Honestly our caching isn't some simple FIFO buffer (we are working on publishing a good 8 page read on it) so unless your having problems and support is directing you to reserve cache, I'd trust ARC and the other systems we use to be smart enough to get the job done.
FYI :The default policy is mirror once, stripe none, no force provisioning, no space reservation.
You can have a lot of fun with policies, but I'm curious what is your use case?