The CIDR block uses a fixed prefix length of /56. Amazon assigns a range of block to the VPC from its pool of IPv6 addresses, but you cannot choose the block for your VPC.
A subnet's IPv6 CIDR block uses a fixed prefix length of /64. For subnet, you can assign an IPv6 CIDR block.
Example: Amazon assigns the following IPv6 CIDR block to your VPC: 2001:db8:1234:1a00::/56. You can create a subnet and associate an IPv6 CIDR block from this range such as 2001:db8:1234:1a00::/64.