ubuntu – Installing OpenVPN to Replace Cisco VPN or Downgrade Kernel

Of course you can run both at the same time. IIRC t is also possible to connect to a cisco concentrator using vpnc, as long as it has ipsec support. What will happen when you do both we can't tell. That depends on the VPN configurations, which may be pushed to your computer from the remote end.

To get what you want you probably have to manually configure your routing table to make sure only the correct traffic goes into the correct endpoint. There is no generic answer to this question though.

/etc/resolv.conf defines how the computer resolves host names (e.g. which, if any, default domain names are searched when you try to resolve a non-FQDN hostname....lookup for a bare www becomes www.yourdomain.example.com), and which name servers are used to do the lookup.

One of the reasons, and the most likely, that VPN clients might modify /etc/resolv.conf is to make the VPN client computer use a particular nameserver for hostname resolution - e.g. if the VPN router runs a caching name-server.

There is nothing stopping you from changing /etc/resolv.conf back to what you want it to be, and the VPN client software may even provide you with some automated method of doing that (otherwise just cp a backup copy of resolv.conf back into place)....but you may have difficulty resolving some names (e.g. some domains are set up with public and private views - outsiders see only "public" names, while insiders - including VPN clients - see all the internal, private names as well).

One way to solve this would be to cp not a backup copy of the original resolv.conf, but a modified copy with both your preferred name-servers and search domains as well as what the VPN client software wants.