For some reason your client is unable to delete the current default route when the tunnel opens, thereby causing two default routes to exist in the routing table.

What you are going to have to do is give the current route a lower metric (higher number) before the tunnel comes up. You can see the metric with the route -n command:

 # route -n 
 Kernel IP routing table
 Destination Gateway Genmask Flags Metric Ref Use Iface
 192.168.4.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1
 192.168.3.0 10.8.1.2 255.255.255.0 UG 0 0 0 tun0
 9.15.64.0 0.0.0.0 255.255.252.0 U 0 0 0 eth0
 0.0.0.0 9.15.64.1 0.0.0.0 UG 0 0 0 eth0

So for example give your wlan interface a metric of 20 and let the tunnel route come up with a metric of zero. Your traffic will all be sent down the tunnel route.

In order to configure the metric on the interface just open the wlan interface file in the /etc/sysconfig/network-scripts directory and add a parameter called METRIC=20.

That should do it. Make sure you verify it afterwards with the route -n command.