If you are initiating vpn client connectivity from behind the 831 to outside you need to allow through the IPsec ports udp 500, udp 4500 and protocol 50 esp.

e.g

access-list 101 permit udp any any eq 500 log

access-list 101 permit udp any any eq 4500 log

access-list 101 permit esp any any log

apply acl-101 to your outbound interface

ip access-group 101 in

Rgds

Jorge