"flat" rather than "hierarchical" routing is the recommended method for ipv6. So having your "edge" router assign a VLAN and handle the IPV6 for your downstream network is the better path anyway.
The only real exception to this is very large organizations or ISPs. If you've got a /32 or bigger you probably want to hierarchically organize it. If you've got a /48 or less you probably want to split it at the edge, rather than doing your own prefix delegation.