As I didn't find another answer :
How does VMC from Unchained interact with feat prerequisites, such as Weapon Specialization, which requires you to be a 4th-level Fighter? Does taking a VMC(Fighter) allow you to take it, or does it lock you out ?
It seems it wouldn't work, as VMC seemingly does not give you anything beyond class features, but I'd like to get another point of view on that question.
With VMC you never actually count as the class your multiclassing into. You basically pay 5 of your feats in a specific pattern to gain 5 of the class features of the class you VMC with.
Example: You play a fighter and VMC with Rogue. As you level the feats you would normally get at Character levels 3, 7, 11, 15 ,19 are replaced with a rogue class feature specified in that classes listing in the book.
But by your 20th character level your still a level 20 fighter. You do not actually count as a rogue at all for feats. You will have those 5 rogue class features (which are not at full strength for the most part) and CAN qualify for feats and such which require only those class features.
Note that most of the class features gained this way are changed for balance and are usually not as potent as the actual classes version but most do level up in power with your character if they level up in the original class.
To answer your example question, if a feat requires you to be a class, VMC does not qualify you as that class. If the feat requires a named class feature that VMC gives you, it does.