If you're familiar with the mysterious author's writing, you likely know that there aren't very many Pynchon novels that would allow for adaptation at all. Though Vice may be the most readily cinematic of his works, saying it's the simplest adaptation is sort of like saying a particular volume of Proust would be the easiest to translate. That is: it's only easy if you're really, really good, and probably not even then. That Anderson was able to not only keep the script faithful to Pynchon's work but also able to let loose a film recognizably his own is no small feat. Frankly, I'm willing to stand up and give the guy a standing ovation for that bit of paring down alone. Applause, PTA, you've done well. You've done so well.
It's not about the story, man, it's about the experience.
More specifically, it's about the theory of the experience. More specifically still, it's about what happens when you allow a totally typical work of genre fiction (in this case the detective story) to collapse under the weight of its parts. This is what the title, in its legal definition, refers to: the hidden defect. That inherent piece of something that allows it to wear itself away, and this is what the film does. It doesn't resolve itself or twist itself into some Hollywood-ending, instead it picks up pieces and indulges itself until it collapses into itself.
Along the way, though, Anderson pulls some stellar decisions from the text that make it the film- even if occasionally incomprehensible - thoroughly entertaining. The film is allowed to lend weight to an instance in one scene and revoke it in the next, and Doc moves through bizarre sequences like a super chill Alice in a 70's freak-scene Wonderland, and the one constant is that we remain by his side as the world and the character spectrum expands and contracts, becomes clear then fogs up again.
Many have opted to compare Inherent Vice to The Big Lebowski, and that's apt only up to a point. Inherent Vice makes formalist moves that tend to disrupt the potentially comedic in favor of a fever dream dismantling. Where the Dude is accidentally drawn into intrigue by virtue of his shared name, Doc is not quite so hapless, and Phoenix plays him to great effect as a guy who's really trying even as he's laid back. He's not the man, but he is interested in a type of righteousness. He means well, and as he falls deeper he oscillates between moments of lucid genius, tries hard to put his mind to big questions, and is an active participant where the Dude is nearly always passively put-out. He's on the case in a big way, sees himself as a true detective, and when he fails it tends to be because of all those scattered inherent vices - in his character, in the situation, in the people and constructs around him.
If you do as Doc does, if you go with the flow and follow the leads where they take you, you should be able to view the film as a wholly worthwhile trip on precisely those terms. It's funny, strange, spellbinding work that runs through rich sequences with an ear for dialogue and an eye for textures and faces that should delight any cinephile. In it's ensemble arrangement it's pure, one-off Paul Thomas Anderson, but at its heart it's a damn fine adaptation of a difficult to pin down fiction. Let it wash over you. It's not about "getting it," it's about seeing it.