entombment vs vassalage

entombment

noun
  • The act of entombing or the state of being entombed 

  • The decommissioning of a radioactive site by encasing it in concrete 

vassalage

noun
  • The state of being a vassal. 

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