ESXi is in fact aware if a LUN is thin without VASA, ESX will identify thin provisioned LUNs using thin provisioning enabled (TPE) bit from READ CAPACITY (16) response. This is of course the arrays are T10 compliant which EMC arrays are.  It is correct that VASA is mainly for administrative purposes, but there are a few other things to note. Namely the fact that there is an extra alert activated by the VASA Provider that passes along an alert when the thin device hits a certain percent full on the array (the thin device itself, not the pool). If this alert is sent along to vCenter any new provisioning on that VMFS is prevented until the situation is cleared (clearing the thin LUN somewhat, increasing the size of the thin lun etc...)  My white paper that dynamox pointed out should answer your other questions.
I created several thin LUNs from VMAX for VMware ESX host because I want to test FAST VP. After adding these LUNs to ESX storage group, some questions arised on how VMware handle thin LUNs.   1. If VASA is not configured, will ESX be aware of this is a thin LUN not? My guess is no. 2. If ESX doesn't know it's thin LUN, will VMFS datastore creation allocate all space of the thin device? My guess is yes 3. Is there a way not to allocate the whole device capacity? Currently I  create a fairly small, 6G VMFS datastore from thin device which can be expanded later. Will only the 6G be allocated? What if we use RDM, does RDM work? 4. If VASA is configured, what different does it make for thin device provision, Datastore creation?  Your inputs are welcomed.
How will ESX handle Virtual Provisioned LUN