I am with OVH and have configured my dedicated server (Not a VPS!) par instructions via:

http://docs.ovh.ca/en/guides-network-ipv6.html#redhat-derivatives-centos-scientific-linux-clearos

As OVH dedicated machines (BHS datacenter at least) do not need fe80 interface I have previously through a ticket ascertained the correct IPv6 information in the past. Even with virtual infrastructure running upon said dedicated machine, when it comes to IPv6 addressing, so long as the address isn't used, things do in such "just work". I was testing ClearOS tonight and configured IPv6 per instructions provided in the docs/guide and got IPv6 working without any issue.

I had the odd experience that while I was trying to get Certbot (Let's Encrypt) working with properly setup ns records pointing to the machine as confirmed by nslookup, dig, ping, etc. that IPv6 all of a sudden flaked out. I had a chat with a ClearOS staffer on the live chat so that I could make my dilemma made aware, and she admitted that many modules in ClearOS don't yet have proper support for IPv6.

I am very suspect that iptables itself has been the culprit as I am not new to these sort of problems, especially considering that ClearOS doesn't yet expose IPv6 configuration to its web interface with the exception of DNS servers. Strangely enough, it started working again after a few minutes, but the sporadic behaviour, which as I alluded to is probably related to the lack of real IPv6 support in the web ui is definitely an area that needs to be addressed since Solaris 7 was the first commercial system to support IPv6 20 years ago as of next year, and it's a crying shame to have a decent system that is for the potential uses with SMB applications and the like curtailed by lack of pervasive support throughout its service offerings, arguably as of yet down to the core.