I'm not aware of a jailbreak that forces only a VPN and the core OS has no provision to only route traffic through a VPN.
The OS needs to contact an insecure network and send traffic in order to establish a VPN connection. For the time the device is connected to the network and not having built a secure tunnel - you are vulnerable to network sniffing. Even once you have a VPN - the traffic can still be sniffed and recorded for later analysis, but the encryption/obfuscation makes it harder to tell what data is being transmitted and to what servers you are communicating since all traffic goes to the VPN server.
In short, you need to be vigilant to ensure the VPN reconnects. If you make sure that your iOS device does not auto-join these untrusted WiFi hotspots, you will have to manually re-join if it drops so that you can manually fire up the VPN again and the device will not inadvertently re-join and put your traffic and data at risk.