My small company uses a 2811 ISR for VPN services (among other tasks such as internet access, p2p circuits to a second site, etc). I have a couple of remote users that have 871 routers that have occasional problems with their routers dropping their VPN tunnels to the 2811. I'm not really sure where to start with the troubleshooting. There are other clients (such as my own 871W) that seem to maintain a connection for weeks. These remote routers that do drop the connection usually reconnect at their next schedule attempt (180 seconds or so.)
Most of the previous questions I've seen similar to this involve software clients but these are hardware routers as the clients and as such I'm not sure how to enable or retrieve logs for the VPN sessions.