I think there could be a bug in the software.
When I use Auth & Priv password for SNMPv3, the Virtual Fabrics are not discovered (files tmp/*vsan* are empty) and the switches appear with incomplete data (only shows the home VF for the user).
However, it does work using NoAuth/NoPriv. It also works using different usernames, and not only "snmpuser1". I haven't tried only Auth or only Priv protocols, always both together which leads to the bug.
So, when I put in place MD5/AES it stops retrieving the VSAN info.