It has become commonly accepted in the 21st century that until the post-war period string players did not use much vibrato – that wiggle of the fingers on the string which produces a quiver of pitch in the note being played. The evidence for this comes almost exclusively from early recordings from the first decades of the 20th century. There is no doubt that string sections back then did not have the same constant vibrato that we tend to hear in present-day performances. But there is a problem with taking that particular historical practice and simply copying it in today’s situation. There are three other, crucial differences in string playing today which have to be taken into consideration.
i) Before the 2nd World War most players used gut rather than steel strings. A gut string has its own internal quiver due to the irregularity of the natural material, whereas steel is naturally clean and ‘cold’ and in need of vibrato to warm up its sound. ...
ii) String players tend to play more in tune today than they did in the early decades of the 20th century and the general standard of rank and file orchestral playing is higher now. As whole violin sections play with a ‘purer’ intonation today there is a reduction in the complexity of colour – and the number of ‘pitches’. Twelve violins, each playing with a slightly different tuning, will simulate a vibrato.
iii) There was a time when not only was pitch less uniform in a string section, but shifting to that pitch was less cleanly executed. Portamento (that gentle, expressive slide from one note to another) was a constantly employed technique. In fact, until the 1940s it is hard to find one bar recorded without one of these inflections. Today things are reversed: you can listen to a whole orchestral concert without hearing one portamento.
--Steven Hough, Telegraph, on better technique necessitating additional compensatory technique. HT: Marginal Revolution