For those with 2920 series routers, who have updated to firmware 3.3.6 (available on the UK site), PDF manual v1.0 has now been superseded by v2.0. The latter manual is not currently available from the UK site so try looking on draytek.com or searching using Google. It contains references to additions including additional fields (i.e. Local Network IP and Local Network Mask) now available on the VPN LAN to LAN page.

Took me a while to find it, so here's a direct link:

http://www.draytek.com/.upload/Rsource/21d47d9650774753bb3c24ee4f6829a220101207055748758589.pdf