vbuckets are exactly one of the ideas proposed in one of the old design document of Redis Cluster. And indeed the final Redis Cluster is using this concept. In our land they are called "Hash Slots".
I think the concept is much more interesting then consistent hashing, as it is a discrete way to see the problem. Dividing the key space into N pieces, and assigning this pieces to different nodes. Resharding this way is easy, just move one hash slot from one instance to another. For moving data around Redis uses the MIGRATE command.
The other subtlety is in being able to prevent the server from servicing requests when it shouldn't be (to avoid consistency issues).
What you don't want is a client not aware of the new configuration -- or reading and writing before the new one is ready. That's where vbucket states allow us to atomically move things.
It's not obvious from your doc how one can actually accomplish this with zero downtime while maintaining consistency.
http://dustin.github.com/2010/06/29/memcached-vbuckets.html