The unique value which identifies this IP address entry for this interface of this virtual router in the Tmnx system. Those row entries with vRiaIndex = 1 represent the primary IP address for the interface identified by the vRtrID and vRtrIfIndex pair. Row entries with other values for vRiaIndex are secondary IP addresses. There must always be a primary IP address assigned an interface. This implies that if the primary address row is deleted, the secondary rows are also deleted.
Sources for this description
Nokia networking MIB copies (LibreNMS/Observium Community)