I noticed your question here is related to Openvpn ipv6 via pppoe. Looking at both together, I think we need to address something important first.
A quick reminder: In our previous thread, I provided you with the complete solution in post #6 (ip6class 'wan_6', script changes, etc.). You then reverted those changes and decided to use the tunnel workaround instead, without fully testing the solution. The current effort with VLAN/radvd/lifetime management is a direct consequence of that decision - the 'simpler' workaround has now created additional complexity. For the future, I'd recommend: when a solution is offered, it's worth implementing and testing it completely before switching to workarounds. This saves time and effort in the long run - for you and for everyone helping.
That said, to provide you with the correct configuration: Which router model and hardware setup do you have? That information would be helpful for a more precise answer.
Looking forward to your response.