Hello lilah,
You are correct. When it comes to just monitoring about the ACI Fabric then you can simply do without VNA. The ACI leaf and spine network is nothing but the Cisco Nexus devices.
However the ACI Infrastructure consist of more than your this. The Application Network Profile, Contracts, End Points, End point groups, etc. These components are not visible via SNMP. And the APIC controller holds all the information and details about the same. And this is where VNA adds significant value and gets all those information and relationships to Spectrum.
I will be more than happy to do a 1-1 session with you on the value addition we bring for Cisco ACI monitoring. Also will give me an opportunity to understand your implementation of Cisco ACI.
Let me know if this works for you.
Thanks,
Nagesh