In the terminal:
sudo nano /etc/ppp/ip-up
Add the lines:
/sbin/route add 10.0.1.0/24 -interface $1
10.0.1.0/24is the subnet you want to route to. The $1 will automatically be populated with the correct interface name.
Add as many routes as required.
Save and exit
sudo chmod a+x /etc/ppp/ip-up
Connect to your VPN, and you route should be created.