The easy way is to just use tunnelbroker.net, that is what I currently have (this would use one of their assigned net blocks, not the one from the VPS). Set it up on the Pi, set up IP forwarding with appropriate firewall rules, make the Pi serve RA so clients can assign themselves an IP, done (IIRC).

If you want to set up the v6/v4 gateway yourself, I would do this with a /64 you can fully route to your home network like you would get with tunnelbroker.net because then you don't have to deal with the network split and essentially two gateways for the same network (your Pi and the VPS), because otherwise your clients would assume the VPS is directly reachable since it's in the same network when in reality it would have to go through the gateway (you would have to set up an extra route in that case on every client, I think). You'd need a second network from Oracle for this.

But it's pretty much the same thing I would assume plus the setup on the VPS side, make the VPN route your /64 block (or use 6in4 which is what tunnelbroker.net uses), configure IP forwarding on the Pi and the VPS between the VPN interface and local/WAN respectively.

You need to set up a publicly accessible device (in this case the VPS) as your IPv6 gateway

So you set up your VPN connecting your network to the VPS (should probably be set up from the router) and set your router to advertise an IP adress for the VPS which is routable from your local network as the gateway address (and should probably also run DHCPv6 for your network)

(note, I have not set up this stuff myself so I can't help with implementation details)