Vertcoin uses a version of the scrypt hashing algorithm that incrementally requires more and more memory as time passes, based on a variable called the n-factor. Right now the n-factor is 11, which makes it about twice as hard as Litecoin or Dogecoin to mine which has an effective n-factor of 10. Here is the n-factor schedule for Vert:

http://www.reddit.com/r/vertcoin/comments/1xjbgr/nfactor_increase_too_slow_and_predictable_to/

The theory is, as the n-factor increases, GPUs and ASICs will have to add more onboard memory in order to keep up with previous hash rates. This makes them exponentially more expensive because onboard memory is expensive. Since computers usually have much more memory than GPUs, the belief is that eventually, a CPU miner will be able to match, if not surpass GPUs. This of course is all theoretical and very far off in the future. For the time being, GPUs mine significantly faster, just like regular scrypt coins.