It looks as if the support for detecting down IPSEC tunnels does not extend to Juniper SRX.
The relevant mib is JUNIPER-JS-IPSEC-VPN-MIB, the oid to walk is jnxJsIpSecTunState, and the feature depends on jnxJsIpSecTunCfgMonState being enabled.
If I provide a box to test on, anyone care to have a go?