Depends on your OS. If you're in linux it will be a couple of iptables rules.
If you're in Windows, it will be an app such as:
- VPN LifeGuard: Open source
- VPNCheck
- VPNetMon
These apps will take your PC offline if the VPN drops, so no leakage of your real IP. I cannot vouch for their reliability or safety in use, becaue I have not used them. I would suggest you research each one for a few minutes just to be sure it fits your needs.
Sorry I didn't link the urls, a quick web search should turn them up quickly for you. Any further queries, I be glad to assist.