Check out the Junos MIB Exporer search.
https://contentapps.juniper.net/mib-explorer/search.jsp
I think the OID for active tunnels you want is:
Name jnxVpnActiveVpns
OID 1.3.6.1.4.1.2636.3.26.1.1.2.0
SyntaxGauge 32
Access read-only
Status current
Description
Number of active VPNs.
I don't see a count for down tunnels but there is a trap for the tunnel going down
Name jnxVpnIfDown
OID 1.3.6.1.4.1.2636.3.26.0.2
Syntax TRAP
Status current
Description
A jnxVpnIfDown notification is generated when the interface with index jnxVpnIfIndex belonging to the VPN named jnxVpnIfVpnName of type jnxVpnIfVpnType transitions to the 'down' state.
I don't see what you are looking for on the interface for VPN. But you might be able to use the jnxVpnIfEntry tree to monitor what you are looking for.
jnxVpnIfTable
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jnxVpnIfEntry
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jnxVpnIfVpnType
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jnxVpnIfVpnName
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jnxVpnIfIndex
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jnxVpnIfRowStatus
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jnxVpnIfStorageType
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jnxVpnIfAssociatedPw
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jnxVpnIfProtocol
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jnxVpnIfInBandwidth
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jnxVpnIfOutBandwidth
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jnxVpnIfStatus