Why can't you subnet the /64? CenturyLink isn't doing dot1q tagging to you, so you must have a router with tagged interfaces on your network.
Edit: Imagine this is because you want to use SLAAC.
Not just because of SLAAC, prefixes smaller than /64 are likely to mess with all sorts of assumptions built into networking gear. It may not be "correct" since there are valid reasons for subnets as small as a /127 (though they are used infrequently enough at least), but a lot of networking gear cheats with IPv6 routes to keep the amount of TCAM memory needed down. Why spend 128-bits per route and quadruple the memory required when you can double it and go 64-bits per route and dump the longer ones to slower DRAM (I mean, you only use /127's for point-to-point links that don't need link-rate switching anyway)?
Regardless of anyone's philosophy of address assignment strategies, prefixes smaller than /64's in IPv6 are un-wise unless you want to degrade into worst-case routing paths on your network gear.
Good example: The SRX300, a desktop router/firewall implemented on a MIPS based Cavium Octeon (both control and data plane) can handle up to 256K IPv6 routes: https://www.juniper.net/assets/us/en/local/pdf/datasheets/10...
The Trident series SOC's can do 8K by default, and 64K with ALPM enabled: https://www.juniper.net/us/en/local/pdf/datasheets/1000480-e...
>(I mean, you only use /127's for point-to-point links that don't need link-rate switching anyway)?
/126's and /127's are often used for PTP's for peering/transit. Line rate is table stakes. Bear in mind that the gear used in these situations has plentiful lookup memory (SRAM, RLDRAM, HMC, or HBM).