I am a very happy guy. Why? Because I just worked out the reason why Entourage doesn’t work.
To be unreasonably succinct: Adrian Grenier is a bad actor.
To be a little bit more elaborative: we are meant to believe that Vincent Chase, Grenier’s character, is a movie star, not only because of his looks but because of his great ability to act. There are various scenes where Chase’s “acting” is used to persuade other characters, and is then described almost reverentially by whoever is witness to it. But the problem with this is that Grenier, in real life, is not a good actor. So now you have a bad actor playing a good actor, which is an unsolvable problem. If it were the other way round, it would be much easier (for example, Johnny Drama is a terrible actor, who is interpreted excellently by Kevin Dillon). But the problem remains.
Now, the confirmation of this theory lies here: if Grenier had performed as a big-time movie star as intended, it would be plausible to expect that his real life would follow suit; life imitating art, if you will. But Grenier sucks as Chase, has sucked in movies, and is probably a B-grade celebrity at best.
Has anyone seen The Devil Wears Prada? Yeah…
Kingculture's Blog 
That’s why finally Adrien doesn’t get paid anywhere near the amount Dillion, Piven and others get paid.
Which is good because honestly, even if he was a great actor, Vinny doesn’t even really feature mainly in the episodes anymore. Which I think is good because the other characters are much more entertaining.
Yeah, I had heard that in the fifth season they learnt to not make the show so Vince-centric, which is good I suppose.