I agree with Louloudi the Centaur about adaptions. Although I do think VDT was a bad adaption and this did add to its being a poor box office success and generally a "whatever" film (because they took out vital pieces of the story that, had they remained, would have left it a good story, because the book really is a good story), I do agree that a bad adaption does not always equal a bad movie, and a movie does not necessarily have to be a good adaption to make good money. The question that rises it, what is a good adaption? In my opinion, the film Pride and Prejudice with Keira Knightly was an excellent adaption. While it did cut out about half the story for the sake of time, nevertheless it tied the ends together well, kept it's pacing steady with interesting dialogue and kept the whole spirit of the book. So I personally don't mind changes as long as the whole spirit of the book is present, and the movie is filmed well.

I personally think VDT's downfalls were among the following
-bad pacing. From the very beginning, the scenes were rushed and confusing.
-bad dialogue. I can't help it, the lines were downright cheesy.
-stereotyped story. They took out major plot lines and substituted it with "typical" Hollywood Fantasy stereo types - the thing every "fantasy" film HAS to have. The result was a waay to predictable piece of junk...and let's face it, few people like to see junk. The problem is not what they cut out, but what they substituted it with.
-junky filming. You just can't compare the quality of the cameras and the graphics with LWW or even PC. The difference is remarkably horrific (maybe I just noticed this because I myself am into filming, graphics, and animation)
I can't comment on the marketing, because here in Russia VDT was a hit despite any lacks in the marketing. That could have been because a Russian pop star sang for the credits, but both ways I don't know much on marketing so I can't say anything about that.

I think a script closer to the book would have been a hit, but it would need to be carefully scripted. It's not impossible to make VDT into a wholesome movie - it just takes delicate work. Look at LOR! They cut out tons of stuff and still managed to make a good quality long lasting adaption! There's a fine balance that needs to be kept in book-to-movie adaptions, and apparently the scriptwriters of VDT were unaware of it.