pfSense » pfSense Plus

What's the end goal you're looking for here?

An IP alias should take a single address you want to add to a specific interface. If instead you need to alias an entire network space for use with NAT or firewall rules, that's done in Firewall > Alias (change Type from host to network).

I need to add a secondary IPv6 address (fd00:0:0:1::/64) on my tun_wg0 interface and it works using the VIPs
If i put fd00:0:0:1::/128 there all the /64 subnet isn't routed to this interface, which is what i need
Is there another way to add additional IP addresses on an interface ?

In IPv6 there is a prefix ID followed by an interface, which replaces the network ID in IPv6. Assigning the prefix ID to an interface is not valid. You should choose a single address in this prefix for IP Aliases, so you should assign the ::1 or another address from the /64.

Kris Phillips wrote in #note-3:

In IPv6 there is a prefix ID followed by an interface, which replaces the network ID in IPv6. Assigning the prefix ID to an interface is not valid. You should choose a single address in this prefix for IP Aliases, so you should assign the ::1 or another address from the /64.

This is not true for IPv6. There is nothing special about the prefix ID address like there is in IPv4. In IPv6 every address in the prefix is usable, including the all zeroes and all ones addresses.