How to cite a dataset, when my calculations relied on the data?
If you are computing something from a dataset, then you need to explain what you have computed and how.  This should resolve the question of citation fairly naturally.
For example, with the example you have given, you might say something like:


  Using the Spanish-language corpus in (Brown, E. K., 2014), n-gram frequency
  computation indicates that
  'E' is the most frequently occurring vowel in the Spanish language and
  appears most often after the letter 's'.


Because the computation is yours, you need to explain and attribute it correctly, as you may have made a mistake in your interpretation, or the dataset itself may be biased.  Thus, it is best to use a few more words and given the reader a clear explanation of the relationships.