Hello johnpoz,
thanks for your quick reply.
You re right, i made a few additional adjustments after i followed the guide mentioned above to fit the setup i need.
To do this i indeed set up the two Google DNS servers (under System -> General setup) which i associated with my regular DHCP_WAN as a gateway. Additionally i created two further DNS entries (the DNS servers of NordVPN) and selected the DHCP_VPN (client) Interface as the gateway this time.
After this i switched to the firewall rules and adjusted every ruleset thats related to "non local" traffic so that LAN and WIFI traffic have the VPN interface set as its gateway and my rules for VLAN100 have the WAN interface as the gateway.
Outbound NAT is still going over WAN for my VLAN100 subnet as well of course.
I am aware that big streamers like netflix and amazon are trying to make it difficult for you to use VPNs and such but what leads me to believe that this might not be the problem here is that if i put my traffic VLAN100 traffic through the VPN i can access amazon and netflix without any trouble.
If i use my WAN as the gateway for my VLAN100 rules several "thatsmyip" websites indicate that there everything is working just as if i wouldnt sue any vpn at all, yet i cant figure out why i run into those problems.