If you have a Kimsufi Dedicated, you got a full /64 not just /128 which is a single IP, lets use it.
  
Next, we need to add IPv6 to our vmbr2, which the VM should be connected to.

  iface vmbr2 inet6 static
       address 2601:xxxx:xx:xxx::2
       netmask 64
       bridge_ports none
       bridge_stp off
       bridge_fd 0
       post-up echo 1 > /proc/sys/net/ipv6/conf/all/proxy_ndp
       post-up echo 1 > /proc/sys/net/ipv6/conf/all/forwarding
       post-up echo 1 > /proc/sys/net/ipv6/conf/default/forwarding
       post-up /sbin/ip -f inet6 neigh add proxy 2601:xxxx:xx:xxx::2 dev vmbr2
       post-up /sbin/ip -f inet6 neigh add proxy 2601:xxxx:xx:xxx::3 dev vmbr0
       post-up /sbin/ip -f inet6 route add 2601:xxxx:xx:xxx::3 dev vmbr2
The next thing you need to do, restart your network on your Dedi with:
   service networking restart
Now Login into your VM and add this to your /etc/network/interfaces:

  iface eth0 inet6 static
       address 2601:xxxx:xx:xxx::3
       netmask 64
       gateway 2601:xxxx:xx:xxx::2
Restart your Networking on the VM:
   service networking restart
If it dosen't respond after a while, try to Reboot your VM over Proxmox.
Now we still need to add IPv6 DNS Server to our /etc/resolv.conf:


  2001:4860:4860::8888
  2001:4860:4860::8844

These are the IPv6 Nameservers from Google but you can use different ones.

If you run:
  ping6 google.com
  
You should get a response and you should be able to reach your VM under this IPv6 which you configured earlier.
   

Sources used: http://hw7.net/?p=252