side piece vs vixen

side piece

noun
  • A mistress; a woman with whom a man carries on a physical affair (not emotional affair) without having the intention of forming a serious relationship. 

vixen

noun
  • A wife who has sex with other men with her husband's consent. 

  • A female fox. 

  • A malicious, quarrelsome or temperamental woman. 

  • A racy or salacious woman who is sexually attractive. 

How often have the words side piece and vixen occurred in a corpus of books? (source: Google Ngram Viewer )