You map each VRF with a specific context name. The context name identifies the VRF and fetches the MIB details of the mapped VRF from the underlying modules. For example, the OSPF-MIB returns the queried OSPF-MIB object values pertaining to the default VRF (default-vrf).
For SNMPv1 and SNMPv2, the mapping of the context is with the community. This mapping is in addition to mapping of the context with the VRF. The SNMP agent supports 256 contexts to support context-to-VRF mapping.
For SNMPv3, you only need to map the context with the VRF. The SNMPv3 request PDU itself provisions for the context. Only one context is allowed for each VRF instance.