2 questions:
1) What exactly is a "hard fork" and how does it prevent ASICs from adapting to the algorithm in-use.
2) How can one tell exactly when ASICs have penetrated VTC mining, vs when GPU's shift to VTC mining because ASICs have forced them out of Scrypt?
1a) A "hard fork" is a change to the source code for the coin that affects the way the blockchain is implemented - such as the hashing algoirthm used in the PoW to solve a block. This fork requires all users of the software to upgrade. This can be done in a large community, within a few weeks - maybe a month at most. Well organised and practiced, could be done in a week or so after implementation.
1b) It takes lots of money and a considerable (12-18 months maybe or so) to research, develop and manufacturer ASICs capable of mining the existing algorithm.
Now if it takes weeks to change, and months as well as hundreds of thousands of pounds or more to re-develop... what Is the incentive?