You can automate the setting of the mtu for the VPN port when the VPN goes up as suggested in script /etc/NetworkManager/dispatcher.d/01-ifupdown. Scripts in the dispatcher.d folder are called with the port and event as arguments.

So, create a script in folder dispatcher.d with the following contents (if you want mtu 1200):

#!/bin/sh  

#info : pre-up and pre-down are not implemented in network-manager

if [ "$2" = "vpn-up" ]; then  
    /sbin/ifconfig "$1" mtu 1200  
fi  

(The name of the script does not matter.) Then make it executable with chmod +x <script>.

-- Anonymous