In all vSAN configurations 1 separate SSD per disk group is used for caching purposes. The other disks in the disk group are used for persistent storage of data.
For all-flash configurations, the flash device(s) in the cache tier are used for write caching only (no read cache) as read performance from the capacity flash devices is more than sufficient.
Two different grades of flash devices are commonly used in an all-flash vSAN configuration: Lower capacity, higher endurance devices for the cache layer and more cost effective, higher capacity, lower endurance devices for the capacity layer. Writes are performed at the cache layer and then de-staged to the capacity layer, only as needed.