Well, open port does not necessary mean that anyone can enter. If you have an open port on a router/modem with nothing listening behind, then there is nothing to compromise.
Of course, this won't let you work from outside your home either. For this to happen, you have to put the VPN server and make it listen to this open port.
What are the vulnerabilities (most probably not exhaustive) :
- a flaw in the router treatment of the IP packets. An intruder sends information with modified content that should have come to the VPN server computer, but instead it got send to a completely different machine (and/or port)
- a flaw in the VPN server software. E.g. buffer overflow and such, which could allow someone to send forged packets to execute specific commands, or a failure in the password validation
- a human flaw : you wrongfully configured your VPN server so he now accepts anonymous connection instead of rejecting them (for example), or you chose a stupidly-easy password as your credentials (admin/admin, florian/password12)
- a flaw in the VPN's protocol, e.g. if poorly designed there could be a way to used wiretapping to recreate an authenticated connection to your VPN.
If you're not a security expert, there are basically no chance that you can find the vulnerabilities by yourself. So what you should do is follow the "best-practices":
- use well-known software that uses well-known protocols. If there is a vulnerability, they will most likely be patched quickly and thus minimise the time-frame of your own network vulnerability
- use good passwords
- do not connect on your home network from computers you can't trust, there could be keyloggers for example
- if you are not confident enough to set things up, perhaps you can find someone you trust to do it for you
If you want to test your own security, there are multiple tools dedicated to this (Nessus, metasploit, ...). They are not really "automated" way to assess the security of one's network, you need expertise to define the tests you want them to perform. There might however bundle some "classic" attack patterns.