vCenter web client 6 offers now the ability to set flash and capacity tier tags for all flash storage options for VSAN. If you use ESXi Embeded Host Client, you can even use Partition Editor to clear partition table/delete existing partitions on the disk(s).
To mark your storage as capacity/flash tier, log in to your vCenter web client, click Host and Clusters, select one host, click tab Manage, then Storage, select device and click either icon HDD or F, depending on the desired tier.
You can confirm that device is set as desired tier in the Device Details section:
From here you can proceed with configuration of your VSAN.