You'd have to parse policies and gateways to determine what VPNs exist, then pull the relevant information based on what you find.
Easiest thing to do is to start with the different VPN Community objects that exist:
From there you can pull the the information from the gateways about what networks and hosts are used and what policy rules may be available.
Note if you're using any Interoperable VPN objects, there is no official API support for these objects until R81.20.
That said, using generic-object calls, you can probably pull the data on these objects.