There are tons of successful vertical search engines, but the line between "search" and an app gets blurry. As examples, Amazon, Octopart, Spotify (if you count music search), the DPLA (a non-profit library search), Shodan, IconFinder, etc.
Also Kayak, HipMunk, Yelp, YouTube, Netflix, AirBnB, Alibaba, etc.
It ceases to be considered "vertical search" and becomes a category in its own right if you do it well enough, even though the way you interact with many of these apps is that you type in free-form text into a search box and it performs some fuzzy matching.
I've explored this in personal projects- a search engine for lectures (https://www.findlectures.com) and stock photos (https://stickstock.com).