One might wonder why VRRP is necessary for IPv6 at all, given that IPv6 includes robust router discovery as part of its Neighbor Discovery (ND) protocol. Section 1.3 of the RFC explains it well (emphasis mine):
Neighbor Discovery (ND) includes a mechanism called Neighbor Unreachability Detection to detect the failure of a neighbor node (router or host) or the forwarding path to a neighbor. This is done by sending unicast ND Neighbor Solicitation messages to the neighbor node. To reduce the overhead of sending Neighbor Solicitations, they are only sent to neighbors to which the node is actively sending traffic and only after there has been no positive indication that the router is up for a period of time. Using the default parameters in ND, it will take a host about 38 seconds to learn that a router is unreachable before it will switch to another default router. This delay would be very noticeable to users and cause some transport protocol implementations to time out.
VRRP, with its sub-second timers, is able to fail over much more quickly than IPv6's built-in neighbor discovery. At the time of this writing, Cisco IOS does not yet support VRRPv3, though it appears likely that it will join Cisco's proprietary first hop redundancy protocols HSRP and GLBP in the near future.
As I've mentioned before, the IETF's New RFCs RSS feed is an excellent way to stay on top of new standards like this as they come out.