If you meant computational efficiency yours is about as good as it gets except perhaps calling .downcase
twice. But really performance wise you will not get much improvement from anything in such a simple case.
If you are looking for a shorter way to write your implementation, this is probably about as short as possible:
def source(url)
url =~ /techcrunch\.com|tcrn\.ch/i && 'vimeo'
end
source('www.stackoverflow.com') # => nil
source('www.techcrunch.com') # => "vimeo"