MMDer Jim Heyworth sent us some excerpts on this topic from "The
Oxford Companion to Music" by Percy A. Scholes, London, Oxford
University Press, 10th ed., 1970, page 1040:

  TREMOLO AND VIBRATO.  These two words seem customarily to be used in
  reverse senses in connexion with stringed instruments and the voice.
  This is unfortunate.  It would be better that both in connexion with
  stringed instruments and voice 'Tremolo' should be the name for the
  effect caused by the tone-generator (bow, breath) and "Vibrato' for
  that caused by the vibrating medium (string, vocal cords), which
  latter involves a fluctuation of pitch.

    (1) In the parlance of the players and teachers of stringed
  instruments "Tremolo' means a rapid iteration of one note by a
  motion of the bow, and "Vibrato' means that effect of waving of the
  pitch by violinists and violoncellists (generally not sufficiently
  markedly to be objectionable, and occasionally to be found indicated
  in notation, as in the following from a Haydn symphony [image].  By
  players of bowed instruments the word 'Tremolo' is also applied to
  two rapidly alternating notes (usually a third apart); this is
  called Fingered Tremolo.

    (2) In the parlance of the vocalists and vocal teachers (when they
  show any knowledge of the existence of two distinct effect, which is
  not always) 'Tremolo' seems usually to mean the wavy, pitch-
  fluctuating effect, and 'Vibrato' the iteration of the one note. [snip]

    To say that these effects are in the twentieth century tremendously
   overdone by a large number of vocalists is to speak mildly. [snip]

    There is a very good psychological case for the abandonment of the
  practice of incessant tremolo or vibrato. [snip]

    The public at large detests the effect of tremolo and vibrato, as
  the numerous letters received by music critics and radio authorities
  testify.  It is said that the tolerance of vibrato in such circles
  dates in Britain from the enormous popularity of Rubini, who spent
  much time in that country between 1836 and 1843. [snip]

I remarked that the author appears to be a weary music critic.

Jim replied, "You should see his critique of 'The Maiden's Prayer'!"

Robbie Rhodes

 [ Please send more evidence of word usage, especially in old text
 [ about mechanical music instruments, to  <editor@foxtail.com> 
 [ Thanks.  -- Robbie