A bit more on this one would be appreciated when you get the opportunity Christo.
From memory, then. Respighi major violin concerto stems from, I guess, 1925 or so (the first one is an early work I don't really care for) and was a failure at its premiere. The composer had high expectations, but was duly disappointed. Yet, even if it's not totally succesful, I think it's one of the cornerstones of his major, 1920s and 1930s style: his very personal brand of '''neo-classicism''', in this case: a rather modal/romantic/neoclassicist version of his ideas about early church music.
But the outcome, the Concerto Gregoriano, is rather beautiful. It reminds a little bit of Vaughan Williams' at his most modal, but is better compared with other attempts by Resphighi in the same direction: especially the String Quartet 'Dorico' from the same period, an passages from his later operas (Maria Egiziaca especially).