Whereas VAAI is used to tell storage what to do and provides offload capabilities, VASA (VMware vSphere Aware Storage API) is used to collect information about storage configuration, capabilities and health status.
Using VASA a storage system can inform vCenter about volumes configuration (RAID level, thin or thick provisioning), snapshot and replication capabilities. It helps system administrators to see storage resources characteristics without using additional tools or configuration worksheets.
The VMware Vsphere 5 Profile Driven Storage feature is based on VASA.
