“I am starting to activate IPv6. As my VM is allocated an IPv6 /64 I’m wondering whether I should use different IPv6 addresses for each site.”
The only reason I can think of to do it is if you want to make SSL/TLS work without warnings for people whose systems suport IPv6 but do not support SNI.
However I strongly suspect that the proportion of users whose systems default to IPv6 and don’t support SNI is a pretty tiny fraction of clients. Users who have manually setup IPv6 on XP and insist on using IE is the only class of user I can think of who would fall into that category.
“Given that BigV only seems to allow setting up RDNS for the main IPv6 address would the lack of RDNS cause a problem?”
Not for webhosting.
The only time i’ve seen lack of reverse dns for an IPv6 address be an issue was when sending email.