Create a physical local virtual disk on the ESXi Host (s) that you want to enable vFlash. Ensure that the local virtual SSD size doesn't exceed the size of the physical ESXi host's local virtual disk.

If you do try to create a partition using Disk Utility in these versions of macOS you will see the warning: “APFS volumes share storage space within a container, occupying a single partition.

You can then also restore your partition from a clone using Disk Utility as well. Also read: How to partition a Mac hard drive or SSD, or create a APFS volume and Best disk partition software for Mac.

You can get higher performance in virtual machines and random I/O operations when using an SSD cache on compatible models.