When I was with Vultr—keyword there being “was”—I simply set up NAT66 for Wireguard to work. I believe that if you want NDP proxying to work you need something like ndppd (https://github.com/DanielAdolfsson/ndppd). Personally, depending on how big of an IPv6 “snob” you are, I would leave Vultr for another VPS/dedicated-server provider that “does IPv6 correctly". While there are some well-respected people in the OpenBSD community—most notably Henning Brauer—that are not big fans of IPv6, I personally find it to be MUCH better than IPv4. To me NDP proxying and NAT are hacks that should be avoided when you can.
IETF recommends (https://datatracker.ietf.org/doc/html/rfc6177) /48 or at least /56 prefixes to be routed to end sites for the purpose of subnetting (e.g., for VPNs), so the fact the Vultr gives out /64s is annoying to say the least. I mean even my residential ISP, Comcast/Xfinity, is willing to give out /60s. I really liked Vultr when I was a customer, but I was too big of a “snob” so I ended up leaving them. I made sure to inform them that I am leaving purely because they only route /64s; and that if they ever changed that, I would seriously consider coming back. While I am sure there are other providers out there that provide more than /64s, I ended up going with a company called ARP Networks. They only have two locations: one in Los Angeles, CA and one in Frankfurt, Germany. They also don’t have nearly as nice of a GUI, but I have been very happy with them. They give you a static /48 when requested. It is also super easy to set up pointer records for both IPv6 and IPv4.
