Based on Mari-Lou A's helpful comment about [checking the ngram references][1], I think it safe to conclude that the peaks (for me, the main reason to think it wasn't a misspelling on vocabulary.com) are caused by the following (unlikely) reasons:
> An author with `et viola` in the name: ([Alberti et Viola][2]).
>
> Books in the English corpus which are (partially) written in Latin. Most of the other results, mostly compilations of prose.



[1]:https://www.google.com/search?q=%22et+viola%22&lr=lang_en&safe=off&nfpr=1&tbs=lr:lang_1en,cdr:1,cd_min:1800,cd_max:1899&tbm=bks&ei=UwwCW_nGBcywsAH3gbuoDA&start=0&sa=N&biw=1920&bih=974&dpr=1

[2]:https://www.google.co.uk/search?safe=off&ei=cQ0CW922DKGMmgW4wrGwCw&q=%22Alberti+et+Viola%22&oq=%22Alberti+et+Viola%22&gs_l=psy-ab.3...10074.11463.0.11667.6.5.1.0.0.0.199.681.1j3.4.0....0...1.1.64.psy-ab..4.1.190...33i160k1.0.Uz2abB1Oq5M