Add following lines to your configuration to make VPN as backup. Suppose on router 1841, fa0 port has the leased line connection and ethernet0 has the vpn map configured and destination network is 192.168.10.0, then commands would be :
ip route 192.168.10.0 255.255.255.0 fa0
ip route 192.168.10.0 255.255.255.0 e0 10