As mentioned before, this seems to be a routing issue. I know that some other VPN client/server are imposing a blocking mode, so that everything actually goes through the VPN. What you want is know as "Split Mode",where part of the routing is going through the virtual adapter, forwarded as encrypted, and the rest goes as usual. Since there seems to be more flexibility under Linux, you should be able (as root) to view your actual routes and change those, or change them in the configuration.
Note that some servers may be able to FORCE a routing to the server, blocking any other route. This may be based on thepolicy and configuration of the VPN server. I use OpenVPN to access a restricted networking in Ottawa for ethical hacking, and the addressing is using non-routable addresses, so the routing is for a specific netmask only. Once I am connected, I can still connect to gmail.com to retrieve my email, while having access to the protected network.
One of the reeasons to do this on VPN is that some organizations do not want to have split connections to avoid infomration leaks that could happen, should ouy have a trojan that could spy on you.