Given two large corpora of text from different sources, is there an accepted way to get differences in vocabulary (n-grams) between them?
As far as I understood your question you can work with NLTK. 

Have a look on the following functions:


Tokenize
Stemmers
pos_tags (if you wanna categorize by Verb, Noun etc. beforehand)


And to come to your question of similarity or relative word/phrase usage you have FreqDistfunction in NLTK. Using this Output you could set each occurance relative to the length of the list that is returned by tokenize