I would personally keep out of the VSX Cluster Management Interface VLAN, I like to keep that just for the actual VSX Management Traffic
SNMPv3 is already encrypted (we always use the option to encrypt anyway) so not sure from a Security perspective too much benefit there and are you really generating enough traffic with SNMP that going to impact regular network traffic.
If you already have a separate Management Network that already using for monitoring/management of other Network Devices etc then would suggest that wouldn't hurt to add an Interface in the VS to that VLAN as already done the majority of the work.
Obviously need to ensure that the SNMP connection wouldn't be reachable by multiple interfaces though of course that would be normal network design practice anyway.