sigh, sad that so many providers failed to understand the goal of IPv6, to do away with IPv4 kludges like masquerading.
This is really not so much an openvpn issue as it is a question for the OS/distro of your VPS. Assuming Linux, you might try looking in the appropriate
ip6tables(8) or
nft(8) documentation, and asking on the Netfilter users mailing list.
What is your end goal, to redirect client IPv6 traffic through the VPN? You could look into a free tunnel service like
HE.net's tunnelbroker service. I don't know how well that would function in the real world, but they've offered it for a long time.
hth, regards, rob0