@Daddi55 

Hey

What you are requesting is not possible unfortunately as these are mutually exclusive settings, one does not affect the other, let me try to explain

Setting up a VPN using OPenVPN allows you to connect to your home network from anywhere at any time, to do this you just need to know your IP address.   Unfortunately, your IP address will change randomly without any actual warning and that is where DDNS comes in, once configured the DDNS will update periodically from your router and map your current IP to an URL   something like   anyone.ddns.net    which is easier to remember than 123.5.2.77 or whatever, should your IP change the DDNS will update its mapping at next checkin.

However you can have a VPN without DDNS, and vice versa.    DDNS in this case will update your IP and create the mapping without a VPN being in place at all.   It certainly wont require the VPN online to work.  

Once DDNS is enabled its for all services, not specifically VPN.   You can use DDNS for game servers, Remote Desktop, Voice etc