I have created a document that details how to setup a route based VPN tunnel from a Juniper SRX to a Draytek 2820.

I did send this to Draytek.co.uk and ask them to put this on the forum, but thus far that have not.

You can download it from
http://www.ilikeit.co.uk/How%20to%20setup%20a%20routed%20IPSEC%20VPN%20tunnel%20from%20Juniper%20SRX%20UTM%20Firewall%20to%20Draytek%202820%20ADSL%20Firewall%20Router.pdf

It also shows how to setup a tunnel from the 2820 to the Juniper SRX.

I hope this helps.