Hmmm, I would think it would work oppositely. If vampire novels become a hot selling ticket, then agents/publishers will want to see more of it because they know they can sell it. Even better if the top bestseller on the list is a vampire novel, then their selling point can be: Hey, if you enjoyed so-and-so novel, then you'll love this, this, that, and this. And, of course, people are going to eat that up. It's the only way I can see why half the YA section is made up of vampire novels.
On the downside, people do get tired of same old, same old (or the bestselling novel that kicked off the trend has turned into an utter joke), and as a result they buy less of that type of novel. Agents/publishers respond by slowing down their production of such novels - they don't stop publishing them, just publish less. It's all about supply and demand.