A typical VPN needs to install a profile on your device. Then it will use that profile to configure VPN connection. I think using this way, as soon as you don't drop your connection, VPN will keep on all the time.
A VPN connection created by Cisco AnyConnect has the best connection stability, based on my experience, but that may requires further configuration to use. Common in enterprise environment.
Uh, I guess my comment could not help you a lot. Sorry.