Arnold Schoenberg (13 September 1874 – 13 July 1951) was an Austrian and later American composer, associated with the expressionist movement in German poetry and art, and leader of the Second Viennese School. He used the spelling Schönberg until his move to the United States in 1934.
photo right from about 1924
We have no direct evidence that Schoenberg was ever 'a vegetarian' - but his interests and connections suggest that the he would have come into close contact with vegetarians, and may well have 'dabbled' himself at some time.
In 'Schoenberg and the Occult', by John Covach, first published in Theory and Practice: Journal of the Music Theory Society of New York State 17 (1992): 103-118, we have the following:
The 1924 Schoenberg 50th birthday Festschrift includes an article by Walter Klein (1924) entitled "Schoenberg as Theosophist."
Most European theosophists at that time were vegetarian, though not necessarily all of them. The American theosophists had broken away and fewer were vegetarian. Schoeberg was still in Germany in 1924.
Covach also shows that Schoenberg made a serious study of Schopenhauer - a major influence on many European vegetarians - and was aware of the writing of Swedenborg, a major influence on the British vegetarian movement of the early 19th century.
We know that Schoeberg became a 'disciple' of Gustav Mahler in 1903, and that Mahler had been vegetarian for some time under Wagner's influence. Then in 1934/5 Schoeberg was teaching John Cage, who became interested in Eastern religions and went on to be a long term vegetarian. Schoenberg and Cage clearly thought highly of each other.
Cage studied with Schoenberg in California: first at the USC and then at the UCLA, as well as privately. The older composer became one of the biggest influences on Cage, who "literally worshipped him", particularly as an example of how to live one's life being a composer.
. . . Although Schoenberg never complimented Cage on his compositions during these two years, in a later interview he said that none of his American pupils were interesting, except Cage: "Of course he's not a composer, but he's an inventor—of genius."
More research is needed to know whether Schoenberg was indeed vegetarian in 1924, along with most other European theosophists, but he would have found it hard to avoid.