step to reproduce:
(in my case but i think it can be reproduced also with different vpn types)

- start first vpnc vpn as default route
- start second openvpn vpn and use it only for resource on his network

when the second vpn start network manager establish the connection to the vpn gateway routing from the first vpn (as default route), but after the vpn connection is established and the routes are pushed to the client, network manager add a route to the second vpn gateway with gateway configured as the isp gateway;
so the public vpn client src ip change for the second vpn and the connection is broken
if we add the route to the second vpn gateway manaually and force it to pass to the isp gateway before starting the second vpn, all work correctly