This object cannot be polled. Its access status is for notification (i.e. traps) only. If you need to find out what VRFs are configured, perform an SNMP Walk of mplsVpnVrfRouteDistinguisher (for example). The VRF names will be on the left-hand side. For example:
MPLS-VPN-MIB::mplsVpnVrfRouteDistinguisher."x" = ""
MPLS-VPN-MIB::mplsVpnVrfRouteDistinguisher."TeSt" = ""
MPLS-VPN-MIB::mplsVpnVrfRouteDistinguisher."marcus" = STRING: "100:1"