(This post assumes that you have a general knowledge of setting up a Juniper device for VPN tunnelling).
The following chain represents my setup:
(internet) --- [Internet router] --- [Juniper Netscreen] ---(LAN)--- [Router] ---(General 10.0.0.0/24 network segment)---
On the Juniper device*, this means that a route of 10.0.0.0/8 is pointing towards Router. In addition, a default route is pointing towards Internet router, allowing for comptuers on LAN (which has the Juniper device LAN IP set as their default gateway) to reach both the 10.x.x.x computers behind Router and to reach internet destinations.
Now I want to create a VPN tunnel over the internet to some other place, tunnelling to that place's internal network 10.10.10.0/24.
Problem: Because of the route of 10.0.0.0/8 is pointing towards [Router], even packets destined for 10.10.10.x-addresses are routed that way. Normally on a Netscreen device, having set up policies for the VPN traffic is enough to route through the tunnel, but routing entries seem to take presedence over policies.
Solution: Set a Metric higher than 1 (e.g. 10) for the 10.0.0.0/8 route. Then create a route of 10.10.10.0/24 pointing towards the [Internet router] IP address (this would be the same as your default gateway points to, of course a public IP address). The 10.10.10.0/24 route will by default receive a lower metric and therefore have priority over the 10.0.0.0/8 route.
Gotcha: Of course, this means that you will not be able to reach 10.10.10.x computers on the network behind Router. You literally can't have it both ways...
* I have done this on a Juniper Netscreen 5xp, but I would assume this would work on all Juniper devices, for instance the successor Juniper SSG.