In case anyone is reading this and Serlin's instructions didn't work you should try using Hamachi:
1. Download the Hamachi VPN
2. Add friends to the network (up to 4 for free).
3. Host a single player game on Vintage Story by selecting 'Open to LAN' then 'Open to Internet'.
4. Copy and Paste the hosts IPV4 address from Hamachi and send to friends.
5. Tell your friends to add the IPV4 to the 'Add New Server' button. Then play.
The host may need to disable their firewall to play (I needed to), besides that I've had no problems hosting a small server for free.