You begin with:
set snmp agent-version v3-Only
set snmp mode vs
set snmp agent on
Then to configure a user:
add snmp usm user <user> security-level authPriv auth-pass-phrase <PW-Auth> privacy-pass-phrase <PW- Privacy> privacy-protocol AES authentication-protocol SHA1
For VSX there are some more options, you can access the VS itself by it's own IP or access all through the VS0 IP with the following command:
set snmp vs-direct-access on/off
To make sure you are allowed to access each VS the following command is used:
set snmp usm user <user> vsid all/<VSID's>
Hope this helps.
Regards, Maarten