How much attention should you pay to the market before you write?

I've been actively watching the YA market for a while now, but never as much as now. I've always been a firm believer in write the idea that grips you, makes you smile and excited to sit at the computer. And I still believe in that to a degree, but there was a point where I would have done so without being really aware of what else was out there.

Now, as I check author/agent/publisher blogs and learn what people in the industry are saying, I realize what a mistake that can be. In YA right now, according to most agents, trying to sell a vampire/werewolf/angel paranormal would be almost impossible. It would take an incredible book to break into that market. Similar with dystopian YA. Two years ago, a dystopian would have been requested by most agents out there, three years ago, vampires were still something publishers were excited about.

As a writer, locked in my writing cave, would I have known that I most likely couldn't sell a vampire book right now. I'm not sure and it is why most agents/publishers say don't write to trends, but be aware of them. I just don't feel as though I've ever been really good at that. Looking at the market and seeing what I shouldn't write. Because like most people who read the Hunger Games, I saw tremendous appeal and potential in a dystopian world, and the potential to create drama in that kind of setting.

So is the answer to be aware of the market? To be aware of what's selling in your genre and ensure that what you are writing is different, but with the elements that still remain true to the essential nature of the genre?

I go back and forth, especially now that I'm submitting, where if you'd asked me a year ago, I would have emphatically stated that the book is the only thing that's important. Now, I'm not entirely sure.