Comment 21 for bug 90681

just a thought:

I use VPN extensively and to multiple endpoints. Before I was using MS VPN client, which had the convenient "use default gateway on remote network" option. This option is absent from network-manager GUI, a similar functionality can be achieved with "only use VPN connection for this addresses" field on routing tab.
That being said, the DNS records in /etc/resolv.conf should be a merge of multiple scenarios - manually entered DNS records (from network-manager manual config), automatic DNS hosts from DHCP, and when connected, DNS hosts from VPN connections. This should be inserted before all other if the connection uses the default gateway, and after all other if it uses only the target LAN IP range.
The mysterious "use peer dns" and "peer dns through tunnel" options seem to be broken as well (Bug #37239), so complete solution is somewhat broken... I would opt for a simpler config interface, maybe with "advanced" tab for all the strange configs geeks use and a set of simple straightforward options for the rest of us ;)