Stangri's VPN Policy Routing package is a mainstay of my OpenWRT builds. It works great for IPv4, but I couldn't, for the life of me, get it working with IPv6. Instead, I had to faux-implement PBR as follows.
My VPN provider only gave me a single IP address with a /128 prefix, so I was forced to use NAT6. This left me with a stanza in my firewall config that looks like this; note the
config zone option name 'wan_vpn' option input 'REJECT' option output 'ACCEPT' option forward 'REJECT'