I know that this is going to be a usecase only realy relevant to me.
I am trying to setup opennds to only work on a vlan, which is already setup and working, but it seems to be working the other way round with my main LAN reciving the captive portal measures and the vlan not.
I have tried many times to get this working but all attempts have failed.
Yes,the solution is to configure your vlan network correctly for openNDS in the first place.
The vlan has to be an ipv4 routed subnet and openNDS has to bind to the bridge interface of your guest network.
I would advise opening an issue on Github where you are more likely to get help.