Baris has an intelligent post on a problem facing vertical search engines:

The promise of vertical search is a good one.  If the search
engine focused on one sector, such as shopping, blogs, electronics or health,
they don’t have to crawl the entire web, and focus on much fewer sites and hone
their algorithms to deliver more relevant results faster.
…
As long as the cost of doing another search on Google is zero, I see a
difficult time for vertical search engines to keep the relevance edge over
time.

I think the real edge vertical search engines hold in the game is the layers of value they add to basic search.  Improved algorithms and site-filtering are good tricks; however, as Baris mentions, they can be manually replicated at no cost by consumers.  Examples of things a consumer can not add are:

• collaborative filtering (including digg-like aggregators, or last.fm-like intelligent proximity algorithms)
• proactive searches (such as RSS feeds indeed brings to you)
• time-related insight (such as historic home price information on Zillow)

I expect most successful vertical engines to keep innovating on the value layers.