drop out vs vow

drop out

verb
  • To leave (school, a race, etc.) prematurely and voluntarily. 

  • Used other than figuratively or idiomatically: see drop, out. 

  • To be lost or momentarily interrupted. 

  • To opt out of conventional society. 

vow

verb
  • To declare publicly that one has made a vow, usually to show one's determination or to announce an act of retaliation. 

  • To make a vow; to promise. 

  • To make a vow regarding (something). 

noun
  • A solemn promise to perform some act, or behave in a specified manner, especially a promise to live and act in accordance with the rules of a religious order. 

  • A declaration or assertion. 

How often have the words drop out and vow occurred in a corpus of books? (source: Google Ngram Viewer )